Handle nil in params setter

All my controller(and acceptance) specs was broken with error
> undefined method `as' for nil:NilClass

I'm using rspec-rails 2.13.2 and rails 4.0.0.

I found out that ActionController::TestCase (rspec uses it) instance fire `#params=` twice. nil first and expected params then.

I am not sure it's acrance bug, but I think we should handle nil and Arcane::Parametrs'able variables