Allows sending on a PULL socket

[mloughran]

There is no error when I call send on a PULL socket, even though this is not allowed (http://api.zeromq.org/2-1-1:zmq-socket#toc13)

The message appears to be buffered by the node code since s.currentSendBacklog() is incremented.

Would it not make sense to raise error when calling send in this case?

[JustinTulloss]

hmm, this is interesting. We only try to send if the socket says that it's writable, but a PULL socket will never be writable, so it'll just sit. I'm ok with that behavior in general, and like you said, you should never be writing to a PULL socket, so I don't really feel the need to fix this.

Let me know if you feel otherwise.