fix delayed job adapter timestamp

Float timestamp doesn't work properly with delayed job (delayed_job 4.0.1). When delayed job tries to save the job into the database (tested with postgresql), it saves the current timestamp (in the run_at field) insted of the expected value.
`lib/delayed/job.rb; line 116`
```ruby
Job.create(:payload_object => object, :priority => priority.to_i, :run_at => run_at)
```
**timestamp.to_f** `(lib/activejob/enqueuing.rb; line 44)`
```ruby
queue_adapter.enqueue_at self, timestamp.to_f, job.job_id, *Arguments.serialize(args)
```
However it works well if it is converted to a Time object, so a quick solution could be use Time.at inside the adapter.
```ruby
Time.at(timestamp)
```