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.