Cache: fix for sending of stale responses.

Cache: fix for sending of stale responses.

For normal cached responses ngx_http_cache_send() sends last buffer and then
request finalized via ngx_http_finalize_request() call, i.e. everything is
ok.

But for stale responses (i.e. when upstream died, but we have something in
cache) the same ngx_http_cache_send() sends last buffer, but then in
ngx_http_upstream_finalize_request() another last buffer is send.  This
causes duplicate final chunk to appear if chunked encoding is used (and
resulting problems with keepalive connections and so on).

Fix this by not sending in ngx_http_upstream_finalize_request()
another last buffer if we know response was from cache.