by calling http_response_header_add().
fix wrong overflow check in spdy_stream_get_header().
html_inject now exports last modified timestamp for the files that are
compiled into the module in the format static_mtime_<type>_<name>.
modules can now look into the request headers using http_request_header_get().
introduce net_send_flush() and net_recv_flush() for this purpose, we attempt to make as much headway as possible until we reach EAGAIN or until we can simply pickup again later.
should merge all the stuff in http_response() into a single send buffer, **out is in place in net_send_queue() for that purpose.