Upstream: fixed cache revalidation with SSI. Previous code in ngx_http_upstream_send_response() used last modified time from r->headers_out.last_modified_time after the header filter chain was already called. At this point, last_modified_time may be already cleared, e.g., with SSI, resulting in incorrect last modified time stored in a cache file. Fix is to introduce u->headers_in.last_modified_time instead.