use of basic auth disables before block

test_rest_api.rb

require 'sinatra/base'

class TestRestApi < Sinatra::Base

  before do
    headers['Access-Control-Allow-Origin'] = '*'
    content_type :json
  end

  # no more header & content_type when to following is enabled
  use Rack::Auth::Basic, "Restricted Area" do |username, password|
    username == 'admin' and password == 'secret'
  end

  # ...

end