Merging of fragments into single unfragmented container fixed

Merging multiple fragment files into a single unfragmented file did not work correctly, e.g. in `MostSimpleDashExample.java`. The media samples were correctly gathered from all fragments, but the movie fragment boxes of the additional fragments were ignored and not used to generate a valid sample table box, resulting in an invalid file that could not be handled by any major software I tested (VLC, MPC-HC, WMP, FFmpeg, Android MediaExtractor).