Rework error status when no matched selectors in CBLRouter

In case of no mached selectors, the CBLRouter will return 405 (method not allowed) if there is an alternative method available otherwise returning 404 (not found). This implementation is in sync with the current SG's implementation. #499
Merge pull request #501 from couchbase/feature/issue_499

Rework error status when no matched selectors in CBLRouter