dashenc: replace attribute id with contentType for the AdaptationSet element

id should be an integer, not a string. It is also optional, so use
contentType instead which is the proper attribute for these values.

This addresses ticket #4545, fixing an MPD validation error.

Signed-off-by: James Almer <jamrial@gmail.com>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
