Executions counting is not a serialization concern

Let’s do it when we actually execute instead. Then the tests dealing
with comparable serializations won’t fail either!

activejob/lib/active_job/core.rb

@@ -83,7 +83,7 @@ def serialize
         'queue_name' => queue_name,
         'priority'   => priority,
         'arguments'  => serialize_arguments(arguments),
-        'executions' => executions + 1,
+        'executions' => executions,
         'locale'     => I18n.locale.to_s
       }
     end

activejob/lib/active_job/execution.rb

@@ -31,6 +31,7 @@ def execute(job_data) #:nodoc:
     def perform_now
       deserialize_arguments_if_needed
       run_callbacks :perform do
+        self.executions = executions + 1
         perform(*arguments)
       end
     rescue => exception