bugfix: G:22 ID3v2 TXXX description encoding

Text encoding of frame description of ID3v2 user-defined text frames
(TXXX / TXX) was not translated to the specified getID3 encoding
(default: UTF-8). Most noticeable if frame encoding was UTF16.

https://github.com/JamesHeinrich/getID3/issues/22
http://getid3.org/phpBB3/viewtopic.php?t=1862