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.
you probably have this property to keep parity with json output formatter where the indent property is not available on settings...to be honest, I am not sure how many people have been using `Indent` behavior in current Web API, but my vote would be is to not surface this up...they can anyway do this themselves by setting this on the writer settings...

Probably we should also look into not surfacing this up on json formatter too and provide some other way...