Makes cancan2.0 compatible with Rails4 and optimise a view call

[gamov]

Optimize DB hits when supplying a scope (AR:Relation)

[gamov]

fixes https://github.com/ryanb/cancan/issues/1000

[xhoy]

Dear submitter, Since cancan/raynB hasn't been active for more than 6 months and no body else then ryam himself has commit permissions the cancan project is on a stand still. Since cancan has several issues including missing support for rails 4 cancan is moving forward to cancancan. More details on: #994

If your feel that your pull request or bug is still applicable (and hasn't been merged in to cancan) it would be really appreciated if you would resubmit it to cancancan (https://github.com/cancancommunity/cancancan)

We hope to see you on the other side!

[gamov]

fixes https://github.com/ryanb/cancan/issues/727