Upstream: no last buffer on errors.

Upstream: no last buffer on errors.

Previously, after sending a header we always sent a last buffer and
finalized a request with code 0, even in case of errors.  In some cases
this resulted in a loss of ability to detect the response wasn't complete
(e.g. if Content-Length was removed from a response by gzip filter).

This change tries to propogate to a client information that a response
isn't complete in such cases.  In particular, with this change we no longer
pretend a returned response is complete if we wasn't able to create
a temporary file.

If an error code suggests the error wasn't fatal, we flush buffered data
and disable keepalive, then finalize request normally.  This allows to to
propogate information about a problem to a client, while still sending all
the data we've got from an upstream.