Add an integration test for sprockets.
user
```ruby
# Frozen-string-literal: true
# Copyright: 2012-2015 - MIT License
# Encoding: utf-8

require "rspec/helper"
describe "Sprockets integration" do
  let( :env) { Jekyll::Assets::Env.new(site) }
  let(:path) { site.in_dest_dir("/assets") }
  let(:site) { stub_jekyll_site }
  before :each, :process => true do
    site.process
  end

  before :all do
    @site = stub_jekyll_site
    @env  = Jekyll::Assets::Env.new(@site)
  end

  it "allows you to merge CSS with sprockets.", :process => true do
    matcher = /^body {\n\s+background-image: "\/assets\/context.jpg";\s+\}/m
    expect(@env.find_asset("merge").to_s).to(match(matcher))
  end
end
```
assistant