Use assert_nil instead as its clearer about what the return is expected to be

activesupport/test/core_ext/object/inclusion_test.rb

@@ -50,6 +50,6 @@ def test_no_method_catching
   
   def test_present_in
     assert_equal "stuff", "stuff".present_in(%w( lots of stuff ))
-    assert_not "stuff".present_in(%w( lots of crap ))
+    assert_nil "stuff".present_in(%w( lots of crap ))
   end
 end