Fixing default value type constructor warnings.

src/ImageSharp/Bootstrapper.cs

@@ -71,7 +71,7 @@ namespace ImageSharp
             Guard.NotNullOrEmpty(format.Extension, nameof(format), "The extension should not be null or empty.");
             Guard.NotNullOrEmpty(format.SupportedExtensions, nameof(format), "The supported extensions not be null or empty.");
 
-            GuardDuplicate(format);
+            this.GuardDuplicate(format);
 
             this.imageFormats.Add(format);
         }

src/ImageSharp/Common/Helpers/Guard.cs

@@ -65,6 +65,7 @@ namespace ImageSharp
         /// <summary>
         /// Verifies, that the enumeration is not null and not empty.
         /// </summary>
+        /// <typeparam name="T">The type of objects in the <paramref name="target"/></typeparam>
         /// <param name="target">The target enumeration, which should be checked against being null or empty.</param>
         /// <param name="parameterName">Name of the parameter.</param>
         /// <param name="message">The error message, if any to add to the exception.</param>

src/ImageSharp/Formats/Jpg/Components/Block8x8F.cs

@@ -381,7 +381,7 @@ namespace ImageSharp.Formats
         internal void Clear()
         {
             // The cheapest way to do this in C#:
-            this = new Block8x8F();
+            this = default(Block8x8F);
         }
 
         /// <summary>

src/ImageSharp/Formats/Jpg/JpegDecoderCore.cs

@@ -1533,12 +1533,12 @@ namespace ImageSharp.Formats
             // blocks: the third block in the first row has (bx, by) = (2, 0).
             int bx, by, blockCount = 0;
 
-            Block8x8F b = new Block8x8F();
+            Block8x8F b = default(Block8x8F);
-            Block8x8F temp1 = new Block8x8F();
+            Block8x8F temp1 = default(Block8x8F);
-            Block8x8F temp2 = new Block8x8F();
+            Block8x8F temp2 = default(Block8x8F);
 
             // Tricky way to copy contents of the Unzig static variable to the stack:
-            StackallocUnzigData unzigOnStack = new StackallocUnzigData();
+            StackallocUnzigData unzigOnStack = default(StackallocUnzigData);
             int* unzigPtr = unzigOnStack.Data;
             Marshal.Copy(Unzig, 0, (IntPtr)unzigPtr, 64);