caf muxer doesn't obey Apple Core Audio Format Specification 1.0, section Information Entry Keys

[nu774]

In ​https://developer.apple.com/library/mac/documentation/musicaudio/Reference/CAFSpec/CAF_spec/CAF_spec.html#//apple_ref/doc/uid/TP40001862-CH210-BCGBAEAD, it is written as:

Apple reserves keys that are all lowercase.
Application-defined keys should contain at least one uppercase character.
Each key can be used only once. You can specify multiple values
for a single key by separating the values with commas.

ffmpeg just directly writes it's internal tag key. It doesn't convert to upcase keys, nor it does respect predefined keys by Apple.

[cehoyos]

Please provide command line etc. to make this a valid ticket.