Issue title: Testing?
 Issue body: If I write a Kemal app, how do I test endpoints?  Sinatra supports Rack38d1:d2b4:3d06:83e8:96a5:8c2:893d:aba5Test so I can do this:

```ruby
describe "sidekiq web" do
  include Rack38d1:d2b4:3d06:83e8:96a5:8c2:893d:aba5Test38d1:d2b4:3d06:83e8:96a5:8c2:893d:aba5Methods

  it "can display scheduled" do
    get "/scheduled"
    assert_equal 200, last_response.status
    assert_match(/found/, last_response.body)
    refute_match(/HardWorker/, last_response.body)

    add_scheduled

    get "/scheduled"
    assert_equal 200, last_response.status
    refute_match(/found/, last_response.body)
    assert_match(/HardWorker/, last_response.body)
  end
end
```
 Comments: 
 Comment 0: Hey there,

I should document this. There's https://github.com/sdogruyol/spec-kemal which does the job but not as complete as Rack38d1:d2b4:3d06:83e8:96a5:8c2:893d:aba5Test
 Comment 1: sorry, I meant to send you a PR to add this but it's not just an edit of an existing page - requires a new doc/ markdown file.
 Comment 2: I'll ping you up when i add the new page then 👍  Thanks!
{'source': 'GithubIssues', 'repo_name_with_owner': 'sdogruyol/kemal', 'issue_id': 158285958, 'repo_stars': 1084.0}