Fix #1039.

Author: jkoritzinsky

Source/SharpDX.MediaFoundation/MediaAttributes.cs

@@ -275,9 +275,9 @@ public unsafe void Set<T>(System.Guid guidKey, T value)
                 return;
             }
 
-            if (typeof(T) == typeof(ComObject) || typeof(T).GetTypeInfo().IsSubclassOf(typeof(ComObject)))
+            if (typeof(T) == typeof(ComObject) || typeof(T).GetTypeInfo().IsSubclassOf(typeof(IUnknown)))
             {
-                Set(guidKey, ((ComObject)(object)value));
+                Set(guidKey, ((IUnknown)(object)value));
                 return;
             }