When storing an object, empty flexible fields are removed from the xxx_attributes DB table.
Cancel this change. Will find another solution.

Revert "When storing an object, empty flexible fields are removed from the xxx_attributes DB table."

This reverts commit c317f573e30b57bd1ef358ee443c750dba939363.
Merge branch 'master' of https://github.com/sampsyo/beets
Merge branch 'master' of https://github.com/sampsyo/beets
Merge branch 'master' of https://github.com/sampsyo/beets
Merge branch 'master' of https://github.com/sampsyo/beets
Merge branch 'master' of https://github.com/sampsyo/beets
Fixed ignored new fields when showing/detecting a model change
Merge branch 'master' of https://github.com/sampsyo/beets
Merge branch 'master' of https://github.com/sampsyo/beets

Conflicts:
	beets/ui/__init__.py
Merge branch 'master' of https://github.com/sampsyo/beets
Merge branch 'master' of https://github.com/sampsyo/beets
Add the ability to gather the result of ^Cveral queries into a smartplaylist
Merge branch 'master' of https://github.com/sampsyo/beets
Merge branch 'master' of https://github.com/sampsyo/beets

Conflicts: kept upstream branch
Added ability to also include in playlist result from query based on albums (album_query parameter).
Merge branch 'master' of https://github.com/sampsyo/beets

Conflicts:
	beetsplug/smartplaylist.py
We can now disable auto regeneration of smartplaylist
Merge branch 'master' of https://github.com/sampsyo/beets

Conflicts:
	beetsplug/smartplaylist.py
Merge branch 'master' of https://github.com/sampsyo/beets