Don't return underlying query builder

Should fix #3732 #3731 and other issues with models being returned as StdClass and undefined methods on query builders.

## How to fix the problem if you have this issue

The only ones who should get this error are people who are actively opting in for non-stable versions of the framework - for example by requiring  `"laravel/framework": "4.1.*"` and having `"minimum-stability": "dev"` in their composer.json. Change your minimum-stability to `"stable"` and `composer update` to fix the issue and not get experimental changes in the future.

Symptoms of this bug are:

- All your Eloquent models are suddenly StdClass objects. `Undefined method StdClass::...` errors.
- Eloquent-specific queries like `with()` no longer work, `Undefined method Illuminate\Database\Query\Builder::with` errors