sdp/media: caps_from_media() don't modify the input media

sdp/media: caps_from_media() don't modify the input media

Performing a gst_sdp_media_get_caps_from_media() would result in
changing fields in the GstSDPMedia violating the const tag in the
function declaration.

Before there would be a line with a=rtpmap:96 VP8/90000
after, that attribute would only contain a=rtpmap:96

Fix by performing modifications on duplicated strings instead of on
the internal values.

Also add a simple test for checking that the representation doesn't
change by a gst_sdp_media_get_caps_from_media()