class Rack::Builder

def warmup(prc = nil, &block)

run MyApp
use SomeMiddleware

end
client.get('/')
client = Rack::MockRequest.new(app)
warmup do |app|

before the Rack application is returned by to_app.
Takes a lambda or block that is used to warm-up the application. This block is called 
def warmup(prc = nil, &block)
  @warmup = prc || block
end