return True from VERIFY_PEER callback if request_client_cert is set

While making some unrelated changes, I noticed I had one failing test, which I eventually tracked down to the callback passed to `SSL.Context.set_verify` when the `request_client_cert` argument to `netlib.tcp.BaseHandler.convert_to_ssl`. According to the documentation for [`SSL.Context.set_verify`](http://pythonhosted.org/pyOpenSSL/openssl-context.html#l2h-179) the callback should return true if verification passes and false otherwise.