chan_websocket: Use leftover data if no frames are available when the timer fires.

chan_websocket: Use leftover data if no frames are available when the timer fires.

When the 20ms channel timer fires but there are no frames available in
the queue, we now check for leftover data in the buffer and if there is
any, we create a frame with it and send it to the core. This resolves an
issue with the leftover data being delayed if a STOP_MEDIA_BUFFERING
command is delayed. Some existing comments were also clarified to
account for the new behavior.

Resolves: #2006