Removes ActiveRecord dependency for #model_hint

Class.name is the same as Class.to_s

(cherry picked from commit bd55756)

Author: gottfrois

lib/jsonapi/resource.rb

@@ -539,10 +539,9 @@ def model_name(model, options = {})
       end
 
       def model_hint(model: _model_name, resource: _type)
-        model_name = ((model.is_a?(Class)) && (model < ActiveRecord::Base)) ? model.name : model
         resource_type = ((resource.is_a?(Class)) && (resource < JSONAPI::Resource)) ? resource._type : resource.to_s
 
-        _model_hints[model_name.to_s.gsub('::', '/').underscore] = resource_type.to_s
+        _model_hints[model.to_s.gsub('::', '/').underscore] = resource_type.to_s
       end
 
       def filters(*attrs)