Relicense with approval from Jose and Miguel. Code snippet
was supposed to be LGPL from the beginning.

https://bugzilla.gnome.org/show_bug.cgi?id=697808#c14
https://bugzilla.gnome.org/show_bug.cgi?id=697808#c15

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

If an attribute is defined without value, the generated text
should be 'a=key' rather than 'a=key:'. For media-level, it
has already been done.

https://bugzilla.gnome.org/show_bug.cgi?id=789742

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()

https://bugzilla.gnome.org/show_bug.cgi?id=769698

<gstsdpmessage.h>
GstCaps*       gst_sdp_media_get_caps_from_media   (const GstSDPMedia *media, gint pt);
GstSDPResult   gst_sdp_media_set_media_from_caps   (const GstCaps* caps, GstSDPMedia *media);
gchar *        gst_sdp_make_keymgmt                (const gchar *uri, const gchar *base64);
GstSDPResult   gst_sdp_message_attributes_to_caps  (GstSDPMessage *msg, GstCaps *caps);
GstSDPResult   gst_sdp_media_attributes_to_caps    (GstSDPMedia *media, GstCaps *caps);

<gstmikey.h>
GstMIKEYMessage * gst_mikey_message_new_from_caps  (GstCaps *caps);
gchar *           gst_mikey_message_base64_encode  (GstMIKEYMessage* msg);

https://bugzilla.gnome.org/show_bug.cgi?id=745880

Fixes https://bugzilla.gnome.org/show_bug.cgi?id=698558

Also added some tests of this improvement.

Fixes https://bugzilla.gnome.org/show_bug.cgi?id=697808