Do we actually need to set streamheader? Are there any elements that need it, or can we just revert 320383c594c4eef821ea380b52021d32c8073056?
As a side note, it would be rather nice to have more tests for the mp4 muxer, ideally tests that use realistic formats like H.264 and AAC, and tests that actually feed samples in and get data out. Testing that the data is correct is likely to be a lot harder, but currently we aren't doing any streaming at all.