BUG/MEDIUM: stream_interface: Don't check if the handshake is done.

BUG/MEDIUM: stream_interface: Don't check if the handshake is done.

In si_cs_send(), don't give up and subscribe if the connection is still
waiting for a SSL handshake. We will never be woken up once the handshake is
done if we're using HTTP/2. Instead, directly try to send data. When using
the mux_pt, if the handshake is not done yet, snd_buf() would return 0 and
we will subscribe anyway.