be more liberal in 0167 payload-type parameter parsing

some implementations will transform the following SDP coming from Firefox

m=audio 12346 RTP/AVP 101
a=rtpmap:101 telephone-event/8000
a=fmtp:101 0-15

to

<payload-type channels="1" name="telephone-event" clockrate="8000" id="101">
  <parameter value="0-15" xmlns="urn:xmpp:jingle:apps:rtp:1"/>
</payload-type>

While a missing name attribute is not legal according to the XEP; and 0-15 are
technically not just one value the following commit will accept it if there is
just one paramater.