Move the png header to constants

src/ImageSharp/Formats/Png/PngConstants.cs

@@ -25,5 +25,16 @@ namespace SixLabors.ImageSharp.Formats.Png
         /// The list of file extensions that equate to a png.
         /// </summary>
         public static readonly IEnumerable<string> FileExtensions = new[] { "png" };
+
+        public static readonly byte[] HeaderBytes = {
+             0x89, // Set the high bit.
+             0x50, // P
+             0x4E, // N
+             0x47, // G
+             0x0D, // Line ending CRLF
+             0x0A, // Line ending CRLF
+             0x1A, // EOF
+             0x0A // LF
+        };
     }
 }

src/ImageSharp/Formats/Png/PngEncoderCore.cs

@@ -167,17 +167,7 @@ namespace SixLabors.ImageSharp.Formats.Png
             this.width = image.Width;
             this.height = image.Height;
 
-            // Write the png header.
-            this.chunkDataBuffer[0] = 0x89; // Set the high bit.
-            this.chunkDataBuffer[1] = 0x50; // P
-            this.chunkDataBuffer[2] = 0x4E; // N
-            this.chunkDataBuffer[3] = 0x47; // G
-            this.chunkDataBuffer[4] = 0x0D; // Line ending CRLF
-            this.chunkDataBuffer[5] = 0x0A; // Line ending CRLF
-            this.chunkDataBuffer[6] = 0x1A; // EOF
-            this.chunkDataBuffer[7] = 0x0A; // LF
-
-            stream.Write(this.chunkDataBuffer, 0, 8);
+            stream.Write(PngConstants.HeaderBytes, 0, PngConstants.HeaderBytes.Length);
 
             QuantizedFrame<TPixel> quantized = null;
             if (this.pngColorType == PngColorType.Palette)

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

@@ -2,9 +2,7 @@
 // Licensed under the Apache License, Version 2.0.
 
 using System.IO;
-using System.IO.Compression;
 using System.Text;
-using SixLabors.ImageSharp.Formats;
 using SixLabors.ImageSharp.PixelFormats;
 using Xunit;
 // ReSharper disable InconsistentNaming