Fix Metacube header handling with multiple header blocks.

Fix Metacube header handling with multiple header blocks.

Some muxes, e.g. MP4, will send multiple header blocks. These are
merged by the HTTP server to a single header which is sent out to
the beginning of each client. However, they are _also_ sent out
directly on the wire to any client that connected before the first
block. In this case, we would send two separate Metacube header
blocks, which would have Cubemap (correctly) discard the first and
set only the second as header.

This would make us send only part of the MP4 header when sending
through Cubemap, if Cubemap connected before the first non-header
block, which would in turn confuse Chrome on Android (although not
Chrome on e.g. Linux). As Cubemap is pretty aggressive about
reconnecting (trying every 200 ms), this could easily happen in practice.