Set default_internal and default_external on AM for testing purposes

actionmailer/test/abstract_unit.rb

@@ -11,6 +11,13 @@ ensure
   $VERBOSE = old
 end
 
+require 'active_support/core_ext/string/encoding'
+if "ruby".encoding_aware?
+  # These are the normal settings that will be set up by Railties
+  # TODO: Have these tests support other combinations of these values
+  Encoding.default_internal = "UTF-8"
+  Encoding.default_external = "UTF-8"
+end
 
 require 'active_support/core_ext/kernel/reporting'
 silence_warnings do
@@ -67,4 +74,4 @@ end
 
 def restore_delivery_method
   ActionMailer::Base.delivery_method = @old_delivery_method
-end
+end