Support argument matchers in have_enqueued_*

[pirj]

Allows for:

have_enqueued_job.at(a_value_within(5.seconds).of(future))
have_enqueued_mail.at(a_value_within(5.seconds).of(future))

Fixes #2205

Time.at seems to be quite fine as per https://github.com/rails/rails/blob/a707072/activejob/lib/active_job/test_helper.rb#L679

@mkamensky I can backport to 3.9 if you like, or appreciate if you do so if this one change gets through.

[glenpike]

Maybe a dumb question, but will this also support have_been_enqueued, e.g.

before do
  #in our before, we actually call AnotherJob.perform_now which calls the following:
  MyJob.set(wait: 7.days).perform_later('123')
end

it 'queues MyJob job' do
  expect(MyJob).to have_been_enqueued.exactly(:once).at(a_value_within(1.minute).of(7.days.from_now))
end

[pirj]

@glenpike Absolutely.
Added a spec for this.

[JonRowe]

Looks good, would prefer it squashed into one commit, and can you add a change log either before or after merging :)

[pirj]

Changelog item added, squashed commits. Forgot to force-push, this pull request almost slipped my mind.