Replacing more instances of hasMutatorMethod

This method no longer exists but the tests didn't seem to show it was failing.

Note: Please review before merging - I did this during a meeting just for convenience.

Broken on commit: https://github.com/laravel/framework/commit/7a87c07e1ba55035987760f7bb2625ab170b2bb2

Broken release: 4.2.12