Don't automatically include ActionController::Serialization

This PR removes some magic I was having a difficult time working around. My legacy app has issues with automatically including ActionController::Serialization in ActionController. Some of my controllers use other methods to render json and the methods defined in this module intercept my calls. 

Since it would be explicitly included if this PR is accepted, we'd probably want a better name for the module. I just left it as ActionController::Serialization for now, but I'm open to suggestions.