email_worker.rb

class EmailWorker
  include SuckerPunch::Worker

  def perform(user_id)
    ActiveRecord::Base.connection_pool.with_connection do
      user = User.find(user_id)

      UserMailer.welcome(user).deliver
    end
  end
end

email_worker_spec.rb

require 'spec_helper'

describe EmailWorker do

  before(:each) do
    @user = FactoryGirl.create :user
  end

  let(:worker){ EmailWorker.new }

  describe "#perform" do
    it "delivers an email" do
      expect{
        worker.perform(@user.id)
      }.to change{ ActionMailer::Base.deliveries.size }.by(1)
    end
  end
end