rtsp: accept any RTSP session id

Makes libcurl work in communication with gstreamer-based RTSP
servers. The original code validates the session id to be in accordance
with the RFC. I think it is better not to do that:

- For curl the actual content is a don't care.

- The clarity of the RFC is debatable, is $ allowed or only as \$, that
  is imho not clear

- Gstreamer seems to url-encode the session id but % is not allowed by
the RFC

- less code

With this patch curl will correctly handle real-life lines like:
Session: biTN4Kc.8%2B1w-AF.; timeout=60

Bug: https://curl.haxx.se/mail/lib-2016-08/0076.html