Add config option to disable automatic active_support conversion

In our application we already have in place a json rendering mechanism in place. 

The idea would be to migrate what we have to AMS but we want to do this in an incremental manner without having to change all the code at once. That and I also prefer to have controller behaviours sandboxed in the controller layer.

This proposed PR allow via an initializer to remove the automagic behaviour around the `render` method. If the `serializer` options is not passed in, this library would not spring into action, leaving the existing code unaffected by the addition of this gem.

What do you think?