Rendering a template from ActionView will default to looking for partials only in the current mime type.

  * The old behavior was tested only as a side-effect of a different test--the original tests remain;
    a new template in the XML mime was added.
  * If you are relying on the current behavior and object to this change, please participate in
    http://groups.google.com/group/rubyonrails-core/browse_thread/thread/6ef25f3c108389bd