Make Resque.enqueue return something deterministic

Currently Resque.enqueue returns an empty [] or the results of the
after_enqueue_hooks that may have been run. If the before_enqueue_hook rejected
the operation we get back a nil.

This commit simply changes enqueue to return nil on rejection and true if the
job is actually queued.

lib/resque.rb

@@ -228,13 +228,15 @@ def enqueue(klass, *args)
     before_hooks = Plugin.before_enqueue_hooks(klass).collect do |hook|
       klass.send(hook, *args)
     end
-    return if before_hooks.any? { |result| result == false }
+    return nil if before_hooks.any? { |result| result == false }
 
-    Job.create(queue_from_class(klass), klass, *args)
+    job = Job.create(queue_from_class(klass), klass, *args)
 
     Plugin.after_enqueue_hooks(klass).each do |hook|
       klass.send(hook, *args)
     end
+
+    return true
   end
 
   # This method can be used to conveniently remove a job from a queue.

test/job_hooks_test.rb

@@ -277,15 +277,15 @@ def self.perform(history)
   test "the before enqueue hook should run" do
     history = []
     @worker = Resque::Worker.new(:jobs)
-    Resque.enqueue(BeforeEnqueueJob, history)
+    assert Resque.enqueue(BeforeEnqueueJob, history)
     @worker.work(0)
     assert_equal history, [:before_enqueue], "before_enqueue was not run"
   end
 
   test "a before enqueue hook that returns false should prevent the job from getting queued" do
     history = []
     @worker = Resque::Worker.new(:jobs)
-    Resque.enqueue(BeforeEnqueueJobAbort, history)
+    assert_nil Resque.enqueue(BeforeEnqueueJobAbort, history)
     assert_equal 0, Resque.size(:jobs)
   end
 end