Silence warning emitted in tests

The instance method `model_name` was being defined multiple times,
causing a redefinition warning.

activemodel/lib/active_model/naming.rb

@@ -216,6 +216,7 @@ def _singularize(string, replacement='_')
   module Naming
     def self.extended(base) #:nodoc:
       base.class_eval do
+        remove_possible_method(:model_name)
         delegate :model_name, to: :class
       end
     end