title: Execute pending job with ActiveJob in rspec
body: <p>I have this code to test ActiveJob and ActionMailer with Rspec
  I don't know how really execute all enqueued job  </p>
  
  <pre><code>describe 'whatever' do
    include ActiveJob::TestHelper
  
    after do
      clear_enqueued_jobs
    end  
  
    it 'should email' do
      expect(enqueued_jobs.size).to eq(1)
    end
  end
  </code></pre>