Added support for Groupdate 4

[ankane]

Hey @codesnik, this adds support for Groupdate 4 (and makes the existing tests pass). I use a similar approach in the ActiveMedian gem. https://github.com/ankane/active_median/blob/8d8313a35117643c60c317c54a0b831457cf0547/lib/active_median/model.rb#L54

Without this, calculate-all returns times instead of dates in keys. This also fills in missing data points.

[ankane]

And here's the process_result code: https://github.com/ankane/groupdate/blob/c74afce40d526cf849a364542ed89130d631a01a/lib/groupdate.rb#L19

It's essentially a no-op if Groupdate wasn't used.

[nicowenterodt]

Thanks @ankane. This works pretty good!

So after testing your PR my code starts working again and the format -options isn't ignored anymore. 🎉

MyModel.group_by_week(:created_at, format: "%W").calculate_all(:average_something)

So I give my Thumbs-Up for merging this one @codesnik

[codesnik]

sorry, guys, I have no time and/or will to even review PR's to that little gem anymore. Maybe I should just pass ownership to someone else.