ext/taglib/gstid3v2mux.cc: Make UTF-8 the default encoding when writing string tags (before, our UTF-8 strings would ...

Original commit message from CVS:
* ext/taglib/gstid3v2mux.cc:
Make UTF-8 the default encoding when writing string
tags (before, our UTF-8 strings would automatically
be converted to ISO-8859-1 by taglib and written as
ISO-8859-1 fields if that was possible).
* tests/check/elements/id3v2mux.c: (utf8_string_in_buf),
(test_taglib_id3mux_check_tag_buffer), (identity_cb),
(test_taglib_id3mux_with_tags):
Add test case that makes sure our UTF-8 strings have
actually been written into the tag as UTF-8.