Fix AR::Relation#cache_key to remove select scope added by user

[prathamesh-sonpatki]

• We don't need the select scope added by user as we only want to max
  timestamp and size of the collection. So we already know which columns
  to select.
• Additionally having user defined columns in select scope blows the cache_key
  method with PostGreSQL because it needs all selected columns in the group_by
  clause or aggregate function.
• Fixes Collection cache_key raises PG::GroupingError with select in query #23038.

r? @matthewd

[prathamesh-sonpatki]

@matthewd I tried investigating group_by and includes queries, but they worked without unscoping select. Can you help with those test cases?

[matthewd]

@prathamesh-sonpatki group(:something) won't cause an error.. it'll just mean we get the count/max for one arbitrary group, rather than the whole relation.

And includes+references / preload will vary the count because of the join.. which may or may not be a thing we want.

I don't have any particular opinion on whether we want/need to address either of those, though.

Maybe we should raise instead of potentially giving misleading answers? ¯_(ツ)_/¯

(but that's all incidental observation, not directly relevant to your fixing select here.)

[prathamesh-sonpatki]

I got it now what you meant. Thanks for the clarification :)

[rafaelfranca]

👍 for raising on group. But this one LGTM. @matthewd !

[prathamesh-sonpatki]

Rebased with master.