Fixed StreamResponse bug with seek position

src/Nancy/Responses/StreamResponse.cs

@@ -27,7 +27,15 @@ private static Action<Stream> GetResponseBodyDelegate(Func<Stream> sourceDelegat
             {
                 using (var source = sourceDelegate.Invoke())
                 {
-                    source.CopyTo(stream);
+                    if (source.CanSeek)
+                    {
+                        source.Position = 0;
+                    }
+
+                    if (source.CanRead)
+                    {
+                        source.CopyTo(stream);
+                    }
                 }
             };
         }