Capitalization not preserved in map_metadata

[darkfeline]

Summary of the bug:

Capitalization is not preserved on map_metadata, at least for MP3 to Vorbis/OGG.

How to reproduce:

Take an MP3 file with the following tag:

MusicBrainz Work Id

Convert it:

$ ffmepg -i input.mp3 -acodec libvorbis -map_metadata:s:a 0:s:a output.ogg

Metadata tag on output file loses capitalization:

musicbrainz work id

Only metadata keys are affected, not metadata value.

"Standard" metadata keys like Title and Artist are not affected.

$ ffmpeg -version
ffmpeg version 2.8.2 Copyright (c) 2000-2015 the FFmpeg developers
built with gcc 5.2.0 (GCC)
configuration: --prefix=/usr --disable-debug --disable-static --disable-stripping --enable-avisynth --enable-avresample --enable-fontconfig --enable-gnutls --enable-gpl --enable-ladspa --enable-libass --enable-libbluray --enable-libfreetype --enable-libfribidi --enable-libgsm --enable-libmodplug --enable-libmp3lame --enable-libopencore_amrnb --enable-libopencore_amrwb --enable-libopenjpeg --enable-libopus --enable-libpulse --enable-libschroedinger --enable-libsoxr --enable-libspeex --enable-libssh --enable-libtheora --enable-libv4l2 --enable-libvidstab --enable-libvorbis --enable-libvpx --enable-libwebp --enable-libx264 --enable-libx265 --enable-libxvid --enable-shared --enable-version3 --enable-x11grab
libavutil      54. 31.100 / 54. 31.100
libavcodec     56. 60.100 / 56. 60.100
libavformat    56. 40.101 / 56. 40.101
libavdevice    56.  4.100 / 56.  4.100
libavfilter     5. 40.101 /  5. 40.101
libavresample   2.  1.  0 /  2.  1.  0
libswscale      3.  1.101 /  3.  1.101
libswresample   1.  2.101 /  1.  2.101
libpostproc    53.  3.100 / 53.  3.100

(Apologies, not development branch, I can't build at the moment, will check again on master when I am able).