SockJS should work cross-origin

Ideally, a SockJS implementation would be configurable with regard to cross-origin requests.  But the the default, IMO, should be to allow them.  Having it hardcoded to disallow cross-origin requests seems counter to one of main features of SockJS, which indeed is to be cross-origin.

This pull request aims to remedy that, by removing the special case altogether.  I open it for discussion.