Added failing integration test for XHR invocation of SessionsController#destroy with `*/*' in the `Accept' header.
Fixed bug in SessionsController#destroy which caused all XHR requests, regardless of `Accept' header, to be treated as 'text/html'.
Merge pull request #2201 from al/bugfix-non_navigational_session_destroy

Fix bug with non-navigational requests being treated as navigational ones in XHR to SessionsController#destroy.