Since mpd 0.19, it has concatenated multiple values using
a ';' character. Mopidy has been using ', '. This makes mopidy
use a ';' for all artist-related values.

In mpd 0.18, multiple values were displayed as multiple lines in
the output, hence this change bumps the mpd protocol version to
0.19 to reflect the new behaviour.

Update tests to reflect new function names

Address issues raised in review:
Fix formatting by shortening function name to concat_multi_values
Change comments and variable names to reflect generic nature of function
Fix typos in tests
Default to single quotes for strings