Update Active Model xml serialization test to reflect a change in builder

Due to a change in builder, nil values and empty strings now generates
closed tags, so instead of this:

    <pseudonyms nil=\"true\"></pseudonyms>

It generates this:

    <pseudonyms nil=\"true\"/>

Document this change in Rails so that people can track it down easily if
necessary.

activemodel/CHANGELOG.md

 ## Rails 4.0.0 (unreleased) ##
 
+*   Due to a change in builder, nil values and empty strings now generates
+    closed tags, so instead of this:
+
+        <pseudonyms nil=\"true\"></pseudonyms>
+
+    It generates this:
+
+        <pseudonyms nil=\"true\"/>
+
+    *Carlos Antonio da Silva*
+
 *   Changed inclusion and exclusion validators to accept a symbol for `:in` option.
 
     This allows to use dynamic inclusion/exclusion values using methods, besides the current lambda/proc support.

activemodel/test/cases/serializers/xml_serialization_test.rb

@@ -133,7 +133,7 @@ def setup
   end
 
   test "should serialize nil" do
-    assert_match %r{<pseudonyms nil=\"true\"></pseudonyms>}, @contact.to_xml(:methods => :pseudonyms)
+    assert_match %r{<pseudonyms nil=\"true\"/>}, @contact.to_xml(:methods => :pseudonyms)
   end
 
   test "should serialize integer" do