Fix signature of GroupChain#size method

[paracycle]

Motivation

Follow up to the comment #2044 (comment) on #2044.

Implementation

Handle the size method separately and make it not take any parameters and return an Integer.

Tests

Updated tests

[iMacTia]

Hey, I don't think this change is correct?
If you call size on a GroupChain (that is, a Relation where you called group) then the result is actually gonna be a hash, like the previous signature said. This change seem to actually introduce a bug?

We realised this while upgrading to tapioca 0.16.8, we have the following code that works in production:

Model
  .select(...)
  .having(...)
  .group(...)
  .size.values.sum

After the upgrade, we get an error like Method "values" does not exist on "Integer", but when I run this locally I can see #size returns an hash like { id => 2 }.
What did you base this change on, if I may ask? Are there other situations where size would return an Integer when called on a GroupChain?