raise on serializing an unsaved model object to a delayed job, refs #1

Right now we serialize the AR object without an id, and fail later when
trying to perform the job, since we can't find the object without an id.
This change will make it a lot easier to track down these kinds of errors.

Change-Id: I89d7fb45182f6624bd7fee9b01c1294112a0ba8b
Reviewed-on: https://gerrit.instructure.com/2354
Reviewed-by: Bracken Mosbacker <bracken@instructure.com>
Tested-by: Hudson <hudson@instructure.com>

vendor/plugins/delayed_job/lib/delayed/backend/active_record.rb

@@ -10,6 +10,9 @@ class ActiveRecord::Base
   end
   
   def dump_for_delayed_job
+    if id.nil?
+      raise("Can't serialize unsaved ActiveRecord object for delayed job: #{self.inspect}")
+    end
     "#{self.class};#{id}"
   end
 end