The current (Edge and 2.0.0 RC 2) Rails will override custom content types for XML, JSON (and JS).
I'm using Rails to generate Atom[Pub] documents where content-type might be something like "application/atomsvc+xml" instead of the MIME::XML standard of "application/xml".
render(
:xml => generate_atompub_service_document,
:content_type => "application/atomsvc+xml")
The path attached here ensures that ActionController::Base respects custom content_type for XML and JSON (not 100% about this last one).
It features a new test method and adds an assertion to a render_xml test to make sure that by default, the content-type is still "application/xml".
This patch might be useful for people working with the Atom[Pub].
