HTTP/2: made it possible to flush response headers (ticket #1743). Response headers can be buffered in the SSL buffer. But stream's fake connection buffered flag did not reflect this, so any attempts to flush the buffer without sending additional data were stopped by the write filter. It does not seem to be possible to reflect this in fc->buffered though, as we never known if main connection's c->buffered corresponds to the particular stream or not. As such, fc->buffered might prevent request finalization due to sending data on some other stream. Fix is to implement handling of flush buffers when the c->need_flush_buf flag is set, similarly to the existing last buffer handling. The same flag is now used for UDP sockets in the stream module instead of explicit checking of c->type.