Upstream: added check if a response is complete.

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.