Fixes undefined method `to_sym' for nil:NilClass error in priority.rb

- Jobs enqueued with Resque.enqueue() are in the default queue with no
priority. When resque-priority is enabled on the class, the call to
perform() fails because the priority is undefined and isn't set to
:normal.

lib/resque/plugins/priority.rb

@@ -29,7 +29,7 @@ def priority_key(*args)
       def around_perform_retrieve_priority(*args)
         key = priority_key(*args)
         priority = Resque.redis.get(key)
-        self.priority = priority && priority.empty? ? :normal : priority
+        self.priority = priority.blank? ? :normal : priority
 
         begin
           yield