combo_handler crashes when combine non-200 response from upstream

I'm sure the crashes appear after TS-1710.

TS-1710 start to save non-200 response(header), while in the past the non-200 response was always empty which is the flag for combo_handler to decide whether to produce 400 BAD REQUEST to user.

The attached patch adds a member of ResponseData for combo_handler(or other ESI client) to get the status code of upstream response.