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:

beets/beetsplug/convert.py

Line 139 in 50bd693

'max_bitrate': 500,

[Tomasito665]

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