jingle: trim media attribute values

Movim has trailing whitespace around some of their media attributes

<source ssrc="1892824964" xmlns="urn:xmpp:jingle:apps:rtp:ssma:0">
  <parameter name="msid" value="{a98821d7-b298-4130-925a-ff6c510734c0} {f45dfc5c-2fa7-42b4-85e5-935e786b3feb} " xmlns="urn:xmpp:jingle:apps:rtp:ssma:0"/>
  <parameter name="cname" value="{63b1042b-5cb5-4411-b2a5-bdff92ae45be}" xmlns="urn:xmpp:jingle:apps:rtp:ssma:0"/>
</source>

our WebRTC doesn’t like that. We trim the value even though this seems to be a Movim bug.