The attribute can be defined without value regardless session-level
or media-level.
Although `gst_sdp_message_insert_attribute` can be used to set NULL,
it would be easier if `gst_sdp_message_add_attribute` accepts NULL.
https://bugzilla.gnome.org/show_bug.cgi?id=789841
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()
