fixed bug where ffmpeg doesn't keep custom http headers when playing hls stream

Problem mentioned here: http://forum.xbmc.org/showthread.php?tid=177341
and here: http://forum.xbmc.org/showthread.php?tid=173484&pid=1524704#pid1524704


Original fix: https://github.com/FFmpeg/FFmpeg/commit/4684539c9148050ad26a8b840c4b5ee33799865d