The [Grape API framework](https://github.com/ruby-grape/grape) regularly
writes tests like
[spec/grape/api_spec.rb](https://github.com/ruby-grape/grape/blob/master/spec/grape/api_spec.rb).

When attempting to write a test in a Rails environment similar to the
following:
```
describe Grape::Api, type: :request do
  let(:app) {
    Class.new(Grape::API) do
      get 'test' do
        { foo: 'bar' }
      end
    end
  }

  it '200s' do
    get 'test'
  end
end
```

The following exception is thrown:

```
NoMethodError: undefined method `url_helpers' for #<Array:0x00007fb4e4bc4c88>
--
0: .../lib/action_dispatch/testing/integration.rb:330:in `block in create_session'
1: .../lib/action_dispatch/testing/integration.rb:326:in `initialize'
2: .../lib/action_dispatch/testing/integration.rb:326:in `new'
3: .../lib/action_dispatch/testing/integration.rb:326:in `create_session'
4: .../lib/action_dispatch/testing/integration.rb:316:in `integration_session'
5: .../lib/action_dispatch/testing/integration.rb:348:in `block (2 levels) in <module:Runner>'
```

This change explicitly ensures that `app.routes` is an
`ActionDispatch::Routing::RouteSet` instance.