Allowing the same stream to both load an save an image

Former-commit-id: a37a9c7ac125eeeff6589778ef3d8626373c59f1
Former-commit-id: dc33ef7ac50d67175796e7cf785bf98ea093099a

src/ImageProcessor/ImageFactory.cs

@@ -1026,6 +1026,8 @@ namespace ImageProcessor
         {
             if (this.ShouldProcess)
             {
+                // Allow the same stream to be used as for input.
+                stream.Seek(0, SeekOrigin.Begin);
                 this.Image = this.CurrentImageFormat.Save(stream, this.Image);
             }