message: Fix compiler error on GCC <= 4.9
diff: @@ -108,7 +108,9 @@ void stream::force_empty_write(const manager_ptr& mgr) {
  void stream::prepare_next_read() {
  collected_ = 0;
  - switch (state_.rd_flag) {
  + // This cast does nothing, but prevents a weird compiler error on GCC <= 4.9.
  + // TODO: remove cast when dropping support for GCC 4.9.
  + switch (static_cast<receive_policy_flag>(state_.rd_flag)) {
  case receive_policy_flag::exactly:
  if (rd_buf_.size() != max_)
  rd_buf_.resize(max_);
  