Please describe the problem:
If x264enc is tried to link with any of the ffmux_* it doesnt link, and gives error "Aspect ratio mismatch between encoder and muxer layer". This comes since x264enc src caps has "pixel-aspect-ratio=(fraction)1/1", while if I use ffenc_mpeg4 it works since the src caps doesnt have any "pixel-aspect-ratio" caps.
On further investigation it revealed that this problem occurs due to a probable bug in ffmpeg ie, in "libavformat/utils.c: (av_write_header):" there is a "av_cmp_q(st->sample_aspect_ratio, st->codec->sample_aspect_ratio)" check that fails. This happens since the aspect ratio incase of x264enc is set in st->codec->sample_aspect_ratio = {1, 1}, while the mux output st->sample_aspect_ratio = {0, 1}. If this check is commented it works fine.

Steps to reproduce:
1. gst-launch-0.10 filesrc location=1.flv ! flvdemux ! ffdec_vp6f ! x264enc ! ffmux_3gp ! filesink location=test.3gp -v
2. gst-launch-0.10 filesrc location=1.flv ! flvdemux ! ffdec_vp6f ! ffenc_mpeg4 ! ffmux_3gp ! filesink location=test.3gp -v



Actual results:
1. Doesnt work, gives error "Aspect ratio mismatch between encoder and muxer layer"
2. Works.

Expected results:
x264enc case should also work.

Does this happen every time?
yes

Other information:
This is valid for any ffmux_*