add controller enabled to test helper

If your controller tests load before your model tests the test.rb
config will cause model test versions to not work even after setting
PaperTrail.enabled to true.

See:
https://github.com/bjallen/paper_trail/blob/6de04f199e7c6c885f922c231872
d04564fb931c/lib/paper_trail/has_paper_trail.rb#L391
Merge pull request #409 from bjallen/master

Documentation Change: add controller enabled to test helper