Improve code coverage.

src/ImageSharp/Formats/Png/Zlib/Deflater.cs

@@ -281,23 +281,15 @@ namespace SixLabors.ImageSharp.Formats.Png.Zlib
 
         /// <inheritdoc/>
         public void Dispose()
-        {
-            this.Dispose(true);
-            GC.SuppressFinalize(this);
-        }
-
-        private void Dispose(bool disposing)
         {
             if (!this.isDisposed)
             {
-                if (disposing)
-                {
-                    this.engine.Dispose();
-                }
-
+                this.engine.Dispose();
                 this.engine = null;
                 this.isDisposed = true;
             }
+
+            GC.SuppressFinalize(this);
         }
     }
 }

src/ImageSharp/Formats/Png/Zlib/DeflaterHuffman.cs

@@ -372,7 +372,25 @@ namespace SixLabors.ImageSharp.Formats.Png.Zlib
         /// <inheritdoc/>
         public void Dispose()
         {
-            this.Dispose(true);
+            if (!this.isDisposed)
+            {
+                this.Pending.Dispose();
+                this.distanceBufferHandle.Dispose();
+                this.distanceManagedBuffer.Dispose();
+                this.literalBufferHandle.Dispose();
+                this.literalManagedBuffer.Dispose();
+
+                this.literalTree.Dispose();
+                this.blTree.Dispose();
+                this.distTree.Dispose();
+
+                this.Pending = null;
+                this.literalTree = null;
+                this.blTree = null;
+                this.distTree = null;
+                this.isDisposed = true;
+            }
+
             GC.SuppressFinalize(this);
         }
 
@@ -407,33 +425,6 @@ namespace SixLabors.ImageSharp.Formats.Png.Zlib
             return code + distance;
         }
 
-        private void Dispose(bool disposing)
-        {
-            if (!this.isDisposed)
-            {
-                if (disposing)
-                {
-                    this.Pending.Dispose();
-                    this.distanceBufferHandle.Dispose();
-                    this.distanceManagedBuffer.Dispose();
-                    this.literalBufferHandle.Dispose();
-                    this.literalManagedBuffer.Dispose();
-
-                    this.literalTree.Dispose();
-                    this.blTree.Dispose();
-                    this.distTree.Dispose();
-                }
-
-                this.Pending = null;
-
-                this.literalTree = null;
-                this.blTree = null;
-                this.distTree = null;
-
-                this.isDisposed = true;
-            }
-        }
-
         private sealed class Tree : IDisposable
         {
             private readonly int minNumCodes;

src/ImageSharp/Formats/Png/Zlib/DeflaterOutputStream.cs

@@ -134,22 +134,20 @@ namespace SixLabors.ImageSharp.Formats.Png.Zlib
         /// <inheritdoc/>
         protected override void Dispose(bool disposing)
         {
-            if (this.isDisposed)
+            if (!this.isDisposed)
             {
-                return;
-            }
+                if (disposing)
+                {
+                    this.Finish();
+                    this.deflater.Dispose();
+                    this.memoryOwner.Dispose();
+                }
 
-            if (disposing)
-            {
-                this.Finish();
-                this.deflater.Dispose();
-                this.memoryOwner.Dispose();
+                this.deflater = null;
+                this.memoryOwner = null;
+                this.isDisposed = true;
+                base.Dispose(disposing);
             }
-
-            this.deflater = null;
-            this.memoryOwner = null;
-            this.isDisposed = true;
-            base.Dispose(disposing);
         }
     }
 }

src/ImageSharp/Formats/Png/Zlib/ZlibDeflateStream.cs

@@ -120,28 +120,16 @@ namespace SixLabors.ImageSharp.Formats.Png.Zlib
         }
 
         /// <inheritdoc/>
-        public override void Flush()
-        {
-            this.deflateStream?.Flush();
-        }
+        public override void Flush() => this.deflateStream.Flush();
 
         /// <inheritdoc/>
-        public override int Read(byte[] buffer, int offset, int count)
-        {
-            throw new NotSupportedException();
-        }
+        public override int Read(byte[] buffer, int offset, int count) => throw new NotSupportedException();
 
         /// <inheritdoc/>
-        public override long Seek(long offset, SeekOrigin origin)
-        {
-            throw new NotSupportedException();
-        }
+        public override long Seek(long offset, SeekOrigin origin) => throw new NotSupportedException();
 
         /// <inheritdoc/>
-        public override void SetLength(long value)
-        {
-            throw new NotSupportedException();
-        }
+        public override void SetLength(long value) => throw new NotSupportedException();
 
         /// <inheritdoc/>
         public override void Write(byte[] buffer, int offset, int count)
@@ -161,17 +149,7 @@ namespace SixLabors.ImageSharp.Formats.Png.Zlib
             if (disposing)
             {
                 // dispose managed resources
-                if (this.deflateStream != null)
-                {
-                    this.deflateStream.Dispose();
-                    this.deflateStream = null;
-                }
-                else
-                {
-                    // Hack: empty input?
-                    this.rawStream.WriteByte(3);
-                    this.rawStream.WriteByte(0);
-                }
+                this.deflateStream.Dispose();
 
                 // Add the crc
                 uint crc = (uint)this.adler32.Value;
@@ -181,6 +159,8 @@ namespace SixLabors.ImageSharp.Formats.Png.Zlib
                 this.rawStream.WriteByte((byte)(crc & 0xFF));
             }
 
+            this.deflateStream = null;
+
             base.Dispose(disposing);
             this.isDisposed = true;
         }

tests/ImageSharp.Tests/Formats/Png/PngEncoderTests.cs

@@ -63,7 +63,7 @@ namespace SixLabors.ImageSharp.Tests.Formats.Png
         /// </summary>
         public static readonly TheoryData<int> CompressionLevels = new TheoryData<int>
         {
-            1, 2, 3, 4, 5, 6, 7, 8, 9
+            0, 1, 2, 3, 4, 5, 6, 7, 8, 9
         };
 
         public static readonly TheoryData<int> PaletteSizes = new TheoryData<int>