Add parentheses to ruby2_keywords method call

For consistency with all the other usages of the method in the code base.

actionview/lib/action_view/test_case.rb

@@ -287,7 +287,7 @@ def method_missing(selector, *args)
           super
         end
       end
-      ruby2_keywords :method_missing
+      ruby2_keywords(:method_missing)
 
       def respond_to_missing?(name, include_private = false)
         begin