Small enhancements

* Update stomp_send() to require the frame body length and add a
  content-length header with that value, (may change at some point to allow or
  require the caller to pass their own headers including this)
* Add a prefetch-count header on subscribe frames, probably only works on
  RabbitMQ but should stop it bombarding the caller with message frames and
  then (rather unfairly) throttling the crap out of send/ack frames