Old pugixml versions didn't recode to UTF-8 themselves. Therefore
mkvmerge contains code to determine the encoding from the XML
declaration's `encoding` attribute and recoding the content to UTF-8
before passing it to pugixml. The old `encoding` attribute was left
untouched.
Newer pugixml versions do recode to UTF-8. In order to prevent
that (and therefore recoding twice), the XML declaration's `encoding`
attribute must be set to `UTF-8`.