Merge pull request #7477 from kennyj/fix_7469

A missing require is added to fix build. When performing rake test:isolated in actionmailer, test/base_test.rb was break.

actionmailer/test/base_test.rb

@@ -2,6 +2,7 @@
 require 'abstract_unit'
 require 'set'
 
+require 'action_dispatch'
 require 'active_support/time'
 
 require 'mailers/base_mailer'