Rails 4 compatibility

[changok]

1. In spec, expecting by .raise_error without error type is deprecated as it can cause false positive. So, I added expected error types and replace unnecessary error expectation to attribute checks
2. In Rails4, there's no difference between .scoped and .all. Both returns active relation. In doing so, we don't need to specify find_scope any more for FinderMethods methods. Also the existing behavior for it doesn't give us any benefit. It just calls the scope at the end of query result.

[CLAassistant]

Thank you for your submission! We really appreciate it. Like many open source projects, we ask that you sign our Contributor License Agreement before we can accept your contribution.
_{You have signed the CLA already but the status is still pending? Let us recheck it.}