Fix WebRTC over WSS not working

Several fixes for the WebSockets implementation in res/res_http_websocket.c

* Flush the websocket session FILE* as fwrite() may not actually guarantee sending
  the data to the network. If we do not flush, it seems that buffering on the SSL
  socket for outbound messages causes issues

* Refactored ast_websocket_read to take into account that SSL file descriptors
  may be ready to read via fread() but poll() will not actually say so because
  the data was already read from the network buffers and is now in the libc buffers

(closes issue ASTERISK-23099)
(closes issue ASTERISK-21930)
Review: https://reviewboard.asterisk.org/r/3248/


git-svn-id: http://svn.asterisk.org/svn/asterisk/branches/11@409681 f38db490-d61c-443f-a65b-d21fe96a405b