Fix for unsafe constructs
Created by: TobbeMobiTV
Replacement of unsafe constructs (innerHTML) with textContent, and filtering of characters in lang attribute in manifest to allowed ones.
Fixes #787 (closed).
Test content: http://async5.org/dash/ED_OnDemand_5SecSeg_Subtitles.mpd http://streamtest.eu/dash/vod/testpic_2s/bug_subs.mpd
The first one is from #787 (closed) for the "lang" attribute. The second is for TTML subtitle text.