Fix break in XmlSerializerOutputFormatter tests

The wrong type is being passed in to the wrapper-provider for
XmlSerializer. DCS formatter is already correct.

src/Microsoft.AspNet.Mvc.Formatters.Xml/XmlSerializerOutputFormatter.cs

@@ -169,7 +169,7 @@ namespace Microsoft.AspNet.Mvc.Formatters
                 if (wrappingType != null && wrappingType != context.ObjectType)
                 {
                     var wrapperProvider = WrapperProviderFactories.GetWrapperProvider(new WrapperProviderContext(
-                        declaredType: wrappingType,
+                        declaredType: context.ObjectType,
                         isSerialization: true));
 
                     value = wrapperProvider.Wrap(value);