Use #model_name on instances instead of classes

This allows rails code to be more confdent when asking for a model name, instead of having to ask for the class.

Rails core discussion here: https://groups.google.com/forum/#!topic/rubyonrails-core/ThSaXw9y1F8
Merge pull request #15889 from carnesmedia/model-name

Use #model_name on instances instead of classes
No need to check model_name anymore