Avoid respond_to? when checking existence of the ruby2_keywords method

Using `respond_to?` causes issues in rspec-rails because of how the `Railtie` class implements `respond_to_missing?`.

Author: bryanp

lib/rspec/mocks/method_double.rb

@@ -63,7 +63,7 @@ def define_proxy_method
           define_method(method_name) do |*args, &block|
             method_double.proxy_method_invoked(self, *args, &block)
           end
-          ruby2_keywords(method_name) if respond_to?(:ruby2_keywords, true)
+          ruby2_keywords(method_name) if Module.private_method_defined?(:ruby2_keywords)
           __send__(visibility, method_name)
         end