Ensure request type is respected with extends

When using `extends` in a template with a custom mime type, the templates lookup is done using the rendered format, rather than the one that was requested. 

This change preserves the request format for the lookup. In the case of the `rabl_test_v1` example, a GET to `/posts` will use `fixtures/ashared/views_rails_3/posts/show.rabl_test_v1.rabl`, rather than `fixtures/ashared/views_rails_3/posts/show.rabl`.

I'm not sure of the use case for `source_format = rendered_format unless rendered_format == :html`, so I may have done something wrong there, some guidence on that would be great :smile: