convert: `max_bitrate` config option should default to none

[ivan-boikov]

Problem

Convert plugin's parameter max_bitrate has default value of 500 (see line 139 in beetsplug/convert.py), but in docs it's none. This results in unexpected copying instead of transcoding. I'd expect none to be a reasonable default.

Setup

Irrelevant

[sampsyo]

Huh, you're absolutely right! Not sure how that default slipped in there: https://github.com/beetbox/beets/blob/50bd693057de472470ab5175fae0cdb5b75811c6/beetsplug/convert.py#L139

[Tomasito665]

I went ahead and gave it a shot. Let me know what you think!