MBS-6019: Extend FreeDB parser to support multiple occurances of the same field

[Some FreeDB responses][550e4807] seem to have fields split into multiple
occurances of the same field. This commit extends the parser to accumulate all
of the fields and concatenate them, rather than using Perl's default behavior
when building a map to take the last field.

[550e4807]: http://www.freedb.org/freedb/classical/550e4807
Merge branch 'mbs-6019' into beta

* mbs-6019:
  MBS-6019: Extend FreeDB parser to support multiple occurances of the same field