Test that all of the supported RDF response types work

The fcr:search endpoint in the code claimed to be able to return application/xml but jena doesn't support serializing Dataset to that content type.  The wiki REST documentation claims other types (application/n3 and application/rdf+json) are supported but they run into the same problem.  These tests check that the supported return types actually can be returned.  The wiki REST documentation needs to be updated to reflect the reality proven by these tests.  Fixes https://github.com/futures/fcrepo4/issues/320.