use assert_includes so we get a useful failure message

activesupport/test/core_ext/hash_ext_test.rb

@@ -505,7 +505,7 @@ def test_one_level_with_nils
     assert_equal "<person>", xml.first(8)
     assert xml.include?(%(<street>Paulina</street>))
     assert xml.include?(%(<name>David</name>))
-    assert xml.include?(%(<age nil="true"></age>))
+    assert_includes xml, %(<age nil="true"></age>)
   end
 
   def test_one_level_with_skipping_types
@@ -513,7 +513,7 @@ def test_one_level_with_skipping_types
     assert_equal "<person>", xml.first(8)
     assert xml.include?(%(<street>Paulina</street>))
     assert xml.include?(%(<name>David</name>))
-    assert xml.include?(%(<age nil="true"></age>))
+    assert_includes xml, %(<age nil="true"></age>)
   end
 
   def test_one_level_with_yielding