Add tests

tests/ImageSharp.Tests/Formats/ImageFormatManagerTests.cs

@@ -2,17 +2,15 @@
 // Licensed under the Apache License, Version 2.0.
 
 using System;
-using System.Collections.Generic;
 using System.IO;
 using System.Linq;
+using Moq;
 using SixLabors.ImageSharp.Formats;
-using SixLabors.ImageSharp.IO;
-using SixLabors.ImageSharp.PixelFormats;
-using SixLabors.ImageSharp.Formats.Png;
 using SixLabors.ImageSharp.Formats.Bmp;
-using SixLabors.ImageSharp.Formats.Jpeg;
 using SixLabors.ImageSharp.Formats.Gif;
-using Moq;
+using SixLabors.ImageSharp.Formats.Jpeg;
+using SixLabors.ImageSharp.Formats.Png;
+using SixLabors.ImageSharp.PixelFormats;
 using Xunit;
 
 
@@ -125,5 +123,24 @@ namespace SixLabors.ImageSharp.Tests
 
             provider.Verify(x => x.Configure(config));
         }
+
+        [Fact]
+        public void DetectFormatAllocatesCleanBuffer()
+        {
+            byte[] jpegImage;
+            using (var buffer = new MemoryStream())
+            {
+                using (var image = new Image<Rgba32>(100, 100))
+                {
+                    image.SaveAsJpeg(buffer);
+                    jpegImage = buffer.ToArray();
+                }
+            }
+
+            byte[] invalidImage = { 1, 2, 3 };
+
+            Assert.Equal(Image.DetectFormat(jpegImage), JpegFormat.Instance);
+            Assert.True(Image.DetectFormat(invalidImage) is null);
+        }
     }
 }