Return an empty collection when where(:foo => []) is given

[chewi]

The previous behaviour was to return everything! This is now consistent with ActiveRecord and avoids a needless request.

The new behaviour is not applied to has_one associations as they currently seem to ignore the where method entirely.

[edtjones]

hi @chewi I've just got commit access on this repo and plan to get back on track with maintenance over time - will be in touch soon!

[chewi]

Rebased.

[chewi]

Rebased again.

[chewi]

And again!