Expose $_SERVER['REQUEST_METHOD'] logic outside of RESTServer->handle()

Please see attached patch.

@ygerasimov I wanted access to the REQUEST_METHOD value in a custom services script (specifically: I was using update to perform validation on delete: related issue: #1691634). Similarly to how drupal core wraps some of the server super globals, like request_uri, I figured this is another good opportunity as there is some logic trapped in REQUEST_METHOD to even allow something like "? _method=DELETE" which would need to be replicated to work properly. Pulling this out to its own method gives others access to this logic.