[reSIProcate] Bug Fixes to SdpContents Class

[Ken Kittlitz]

Hi all,

I've checked in some changes to the SdpContents class to help it deal 
with "static" codecs -- those with payload numbers 96 and below which 
are defined in RFC 3551.  These need not
have rtpmap attribute lines associated with them, but the class was 
assuming that all codecs specified in a media line would have such 
attributes.  Thus, it was possible to lose codecs if you used the class 
to make a copy of an SDP block.

I made a couple of other bug fixes, but that was the big one.  Please 
let me know if you encounter any problems with the class.
    -Ken