default an explicitly passed priority of nil to the default priority

lib/delayed/backend/base.rb

@@ -31,7 +31,7 @@ module ClassMethods
         # :priority, :run_at, :queue, :strand, :singleton
         # Example: Delayed::Job.enqueue(object, priority: 0, run_at: time, queue: queue)
         def enqueue(object,
-                    priority: default_priority,
+                    priority: nil,
                     run_at: nil,
                     expires_at: nil,
                     queue: Delayed::Settings.queue,
@@ -44,6 +44,7 @@ def enqueue(object,
             raise ArgumentError, "Cannot enqueue items which do not respond to perform"
           end
 
+          priority ||= default_priority
           strand ||= singleton if Settings.infer_strand_from_singleton
 
           kwargs = Settings.default_job_options.merge(kwargs)

spec/shared/shared_backend.rb

@@ -99,6 +99,16 @@ def no_op_callback(_); end
     Delayed::Job.default_priority = 0
   end
 
+  it "uses the default priority when a priority of nil is explicitly specified" do
+    Delayed::Job.default_priority = 0
+    @job = Delayed::Job.enqueue SimpleJob.new, priority: nil
+    expect(@job.priority).to be 0
+    Delayed::Job.default_priority = 10
+    @job = Delayed::Job.enqueue SimpleJob.new, priority: nil
+    expect(@job.priority).to be 10
+    Delayed::Job.default_priority = 0
+  end
+
   it "is able to set run_at when enqueuing items" do
     later = Delayed::Job.db_time_now + 5.minutes
     @job = Delayed::Job.enqueue SimpleJob.new, priority: 5, run_at: later