Job payload should be symmetric across JSON dump/load

Placing non-native JSON data types, like symbols, in the hash to serialize means that the deserialize method will return something different from what was serialized, a common bug and source of frustration for devs.

activejob/lib/active_job/core.rb

@@ -79,7 +79,7 @@ def serialize
         'queue_name' => queue_name,
         'priority'   => priority,
         'arguments'  => serialize_arguments(arguments),
-        'locale'     => I18n.locale
+        'locale'     => I18n.locale.to_s
       }
     end
 

activejob/test/cases/job_serialization_test.rb

@@ -15,13 +15,13 @@ class JobSerializationTest < ActiveSupport::TestCase
   end
 
   test 'serialize includes current locale' do
-    assert_equal :en, HelloJob.new.serialize['locale']
+    assert_equal 'en', HelloJob.new.serialize['locale']
   end
 
   test 'deserialize sets locale' do
     job = HelloJob.new
-    job.deserialize 'locale' => :es
-    assert_equal :es, job.locale
+    job.deserialize 'locale' => 'es'
+    assert_equal 'es', job.locale
   end
 
   test 'deserialize sets default locale' do