Changes to enable XML Output Formatters.

Functional tests for Output Formatters are not a part of this PR. Will follow up with Functional Tests after @harshgMSFT's conneg check-in.
The formatter should override this if formatter needs to look at something other than the contentType, (like the declaredType etc.) .. also typically even in overriding it will call the base implementation to do the supportedMediaType match.