Fix problem with undefined status code method for Rack::MockResponse

Hi.

This patch fixes problem for `expect_status` Airborne DSL method. 

When used with Rack application, `expect_status` compares status code passed to that method with value returned by `response.code` method. That method does not exist in `Rack::MockRequest`. 

https://github.com/brooklynDev/airborne/blob/master/lib/airborne/request_expectations.rb#L34

There is `status` method in `Rack::MockRequest` instead.

This commit adds method `code` that is alias for `status` in `Rack::MockResponse`

Kind regards,
 Grzegorz