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.
Based on current Web API's xml media type formatter, this could throw in case the serializer doesn't support the given type...as in Web API should there be a check in `CanWriteResult`?...yeah, Json formatter doesn't do this check and it eagerly says true for all the types...the side effect of this here is that we do not let other formatters which can indeed handle the type...