Only inject gon into ActionController::Base-like object in spec_helper

This is related to #141. The previous implementation would execute on any controller, but gon only injects itself into `ActionController::Base`. This is particularly damaging if you have a mixed environment that has other base ActionControllers (i.e. you have an api that bases from `rails-api`) since it tries to execute methods that weren't injected onto it.

I've simply added a check to ensure that objects are of type ActionController::Base before proceeding to setup gon for rspec.