Call Bullet.enable=true multiple times

The strategy of method aliasing may create stack level too deep errors because we lost track of the original methods in case the orm patches are evaluated multiple times.

I wanted to enable Bullet only within some of my specs in my test suite. Then, I did it by setting the attribute accessors n_plus_one_query_enable, unused_eager_loading_enable, and counter_cache_enable. But It should be more clear to use `Bullet.enable = false` directly.

If you think it's the way to go, I could add some specs.