gst/matroska/matroska-mux.c: Fix mpeg4 input handling (#318847); also, while we're at it, fix media type for Motion-J...

Original commit message from CVS:
Reviewed by: Tim-Philipp Müller  <tim at centricular dot net>
* gst/matroska/matroska-mux.c:
(gst_matroska_mux_video_pad_setcaps),
(gst_matroska_mux_audio_pad_setcaps):
Fix mpeg4 input handling (#318847); also, while we're at it,
fix media type for Motion-JPEG: should be image/jpeg.