Fixing assignment bug in Eloquent model isset method

After submitting #6702 last night I ran into another bug that has apparently been around [since October](https://github.com/laravel/framework/commit/7a87c07e1ba55035987760f7bb2625ab170b2bb2). It's just a minor syntax bug, but it surprisingly throws a pretty major exception in all cases where `getMutatorMethod()` returns something truthy. I added a test to make sure this doesn't happen again.