h264parse: sets parsed=true on output even in passthrough mode

Submitted by Matej @Knopp

Link to original bug (#737487)

Description

This doesn't seem right. Just because both input and output are in same format, the parser should claim that it has parsed the input and then enable passthrough.

In may case matroskademux output AVC H.264 (unparsed), decoder requires AVC with parsed=true and parser just appends parsed=true to the caps without even looking at the stream.