Media element source node example plays at half speed and half pitch

[aschey]

When running the example media_element_source_node, the playback appears to play at half speed and half pitch. I was able to reproduce this on both Windows and Linux. I poked around briefly and it seems like the problem is related to dual channel audio. I changed the caps for the appsink that it's using to use a single channel and that fixed the problem, but that obviously causes the audio to play in mono instead of stereo, which isn't desirable. Here's the small change I made to demonstrate the change in behavior: aschey@c01e07f