Fix sending of next item in notification queue.

The notification queue used for websocket client notifications had a
bug which caused the next queued item in some situations to not be
sent, but instead send a previously sent item.  This change fixes this
by setting the 'next' variable to the next item which must be
dequeued, if the queue length is non-zero.

This bug did not always manifest itself as if the receiving goroutine
was ready, a queued item could be sent directly to it rather than
waiting in the queue to be sent at a later time.