Draft: basetsmux: Don't propagate buffer's _HEADER flag

The flag _HEADER only has a meaning for a given media type. When a stream is transformed in a different media type like muxer do, we can't consider that what was a header before the muxer is still a header because for the new media type _HEADER has a new meaning and is the muxed stream header. As an example with mpegts PAT & PMT tables prefix the stream (and periodically inserted) and _HEADER flag indicate there presences.

Fixes: #4098 (closed)