ActiveJob Backburner adapter: fix priority

The priority wasn't being passed from ActiveJob to Backburner, despite
priority being supported. This also brings it inline with the docs,
which mark Backburner as supporting priorities in the "Backend Features"
table: https://api.rubyonrails.org/classes/ActiveJob/QueueAdapters.html

activejob/lib/active_job/queue_adapters/backburner_adapter.rb

@@ -16,12 +16,12 @@ module QueueAdapters
     #   Rails.application.config.active_job.queue_adapter = :backburner
     class BackburnerAdapter
       def enqueue(job) #:nodoc:
-        Backburner::Worker.enqueue JobWrapper, [ job.serialize ], queue: job.queue_name
+        Backburner::Worker.enqueue(JobWrapper, [job.serialize], queue: job.queue_name, pri: job.priority)
       end
 
       def enqueue_at(job, timestamp) #:nodoc:
         delay = timestamp - Time.current.to_f
-        Backburner::Worker.enqueue JobWrapper, [ job.serialize ], queue: job.queue_name, delay: delay
+        Backburner::Worker.enqueue(JobWrapper, [job.serialize], queue: job.queue_name, pri: job.priority, delay: delay)
       end
 
       class JobWrapper #:nodoc:

activejob/test/integration/queuing_test.rb

@@ -137,4 +137,16 @@ class QueuingTest < ActiveSupport::TestCase
     assert job_executed "#{@id}.2"
     assert job_executed_at("#{@id}.2") < job_executed_at("#{@id}.1")
   end
+
+  test "should run job with higher priority first in Backburner" do
+    skip unless adapter_is?(:backburner)
+
+    jobs_manager.tube.pause(3)
+    TestJob.set(priority: 20).perform_later "#{@id}.1"
+    TestJob.set(priority: 10).perform_later "#{@id}.2"
+    wait_for_jobs_to_finish_for(10.seconds)
+    assert job_executed "#{@id}.1"
+    assert job_executed "#{@id}.2"
+    assert job_executed_at("#{@id}.2") < job_executed_at("#{@id}.1")
+  end
 end