Fix crash when using sessionless requests over WebSockets.

When disabling sessions for individual requests, it's done by deleting
the session field from the request. (See
https://github.com/balderdashy/sails/issues/841).

Unfortunately, the REST-over-WebSocket code assumed that the session was
always there. This patch checks for the existence of req.session before
calling req.session.save.
Merge pull request #2107 from leedm777/fix-sessionless-websockets

Fix crash when using sessionless requests over WebSockets.