Comment tag in flac files is not imported correctly

If you export a flac file in Audacity (with a comment tag) and then re-import it, the comment field is not picked up. Perhaps due to confusion over whether the tag is called "comment" or "comments": the tag is saved as COMMENTS. Minor impact I realise, but annoying if you use Audacity to edit lots of flac files, as I do.

Steps to reproduce the bug:
- Create a snippet of audio in audacity
- Export as flac. Fill in the comments field and (for example) the artist field.
- Now open the created flac file in Audacity
- Export it to flac again. You will see that the artist field is pre-filled from the saved tags, but the comment field is blank.

Expected behaviour: comment field should be pre-filled just like the other fields.

ProblemType: Bug
DistroRelease: Ubuntu 16.10
Package: audacity 2.1.2-1build1
ProcVersionSignature: Ubuntu 4.8.0-41.44-generic 4.8.17
Uname: Linux 4.8.0-41-generic x86_64
ApportVersion: 2.20.3-0ubuntu8.2
Architecture: amd64
CurrentDesktop: Unity
Date: Thu Mar 23 15:53:12 2017
SourcePackage: audacity
UpgradeStatus: Upgraded to yakkety on 2016-10-14 (159 days ago)