Add json_ensure_response

This ensures that a value is either a WP_JSON_ResponseInterface object,
or a WP_Error, by wrapping any other value in a WP_JSON_Response
instance.
Merge pull request #154 from WP-API/ensure-response

Add json_ensure_response function to ensure either a WP_JSON_ResponseInterface or a WP_Error object is returned.  Fixes #151.