Fix to_yaml test when run individually

The to_yaml method is undefined when running the test as:

  $ ruby -I lib:test test/core_ext/string_ext_test.rb

Doesn't fail when running rake test:isolated presumably because
something else has required 'yaml' already.

activesupport/test/core_ext/string_ext_test.rb

@@ -10,6 +10,7 @@
 require 'active_support/core_ext/string/output_safety'
 require 'active_support/core_ext/string/indent'
 require 'time_zone_test_helpers'
+require 'yaml'
 
 class StringInflectionsTest < ActiveSupport::TestCase
   include InflectorTestCases