Addresses hook.py ability to handle non-string item properties

[broddo]

This is a simple fix for #2740 that catches an AttributeError if the value returned by convert_field() doesn't have a decode() method.

[sampsyo]

Thanks for getting this started! How about something like this to make it clear what types are involved?

if isinstance(converted, bytes):
    return converted.decode(self._coding)
return converted

Would that cover the same set of cases?

[broddo]

That certainly looks better than my approach of "break the rules, then ask for forgiveness" approach! I'll run some tests on the various item properties and get back to you.

[broddo]

Yeah, that works just fine. A config file using this:

    hooks:
        - event: write
          command: echo "writing track {item.title}, number {item.track} and replay track gain of {item.rg_track_gain}"

results in an output like this:

writing track Moonshine, number 12 and replay track gain of -9.07

So it looks like its handling ints and floats properly now.

[sampsyo]

Perfect! Thank you! Would you mind adding an entry to changelog.rst summarizing the fix?

Also, I know this is really small, but there's some extra whitespace that Travis is complaining about: https://travis-ci.org/beetbox/beets/jobs/305090699#L1099

[sampsyo]

This looks perfect! ✨ Thank you again!