Fix a bug in the offset calculation of .enqueue_at.

The old offset calculation was always one second off:

```ruby
irb(main):049:0> offset = ((Time.now + 2) - Time.now)
=> 1.999996
irb(main):004:0> offset.to_i
=> 1
```

Report in rails/rails@40ff508#commitcomment-10860316

changelog

@@ -1,3 +1,6 @@
+Unreleased
+- Fixed a bug in the offset calculation of `.enqueue_at`.
+
 Version 3.0.0rc
 - Improved signal handling
 

lib/queue_classic/queue.rb

@@ -47,7 +47,7 @@ def enqueue(method, *args)
     # and args argument must be in the form described in the documentation for
     # the #enqueue method.
     def enqueue_at(timestamp, method, *args)
-      offset = Time.at(timestamp) - Time.now
+      offset = Time.at(timestamp).to_i - Time.now.to_i
       enqueue_in(offset, method, *args)
     end
 

test/queue_test.rb

@@ -28,12 +28,14 @@ def test_lock_when_empty
   end
 
   def test_lock_with_future_job_with_enqueue_in
+    now = Time.now
     QC.enqueue_in(2, "Klass.method")
     assert_nil QC.lock
     sleep 2
     job = QC.lock
     assert_equal("Klass.method", job[:method])
     assert_equal([], job[:args])
+    assert_equal((now + 2).to_i, job[:scheduled_at].to_i)
   end
 
   def test_lock_with_future_job_with_enqueue_at_with_a_time_object
@@ -44,6 +46,7 @@ def test_lock_with_future_job_with_enqueue_at_with_a_time_object
     job = QC.lock
     assert_equal("Klass.method", job[:method])
     assert_equal([], job[:args])
+    assert_equal(future.to_i, job[:scheduled_at].to_i)
   end
 
   def test_lock_with_future_job_with_enqueue_at_with_a_float_timestamp