Upstream: added check if a response is complete. Checks were added to both buffered and unbuffered code paths to detect and complain if a response is incomplete. Appropriate error codes are now passed to ngx_http_upstream_finalize_request(). With this change in unbuffered mode we now use u->length set to -1 as an indicator that EOF is allowed per protocol and used to indicate response end (much like its with p->length in buffered mode). Proxy module was changed to set u->length to 1 (instead of previously used -1) in case of chunked transfer encoding used to comply with the above.