gst/rtp/: Various class and caps fixes from Andre Magalhaes (andrunko)

gst/rtp/: Various class and caps fixes from Andre Magalhaes (andrunko)

Original commit message from CVS:
* gst/rtp/gstrtpamrenc.c: (gst_rtpamrenc_setcaps):
* gst/rtp/gstrtpgsmparse.c:
* gst/rtp/gstrtph263penc.c:
* gst/rtp/gstrtpmp4venc.c: (gst_rtpmp4venc_class_init),
(gst_rtpmp4venc_parse_data), (gst_rtpmp4venc_handle_buffer),
(gst_rtpmp4venc_set_property):
* gst/rtp/gstrtpmpaenc.c: (gst_rtpmpaenc_handle_buffer):
Various class and caps fixes from Andre Magalhaes (andrunko)