The APIC description must be unique, and some ID3v2 tag writers add
spaces to write several APIC entries with the same description. The
trailing spaces simply serve as a way to disambiguate the description.
Do this so that API users do not have to special-case mp3 to fix this
cosmetic issue.

Requested-by: wm4
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>