Upstream: fix $upstream_cache_last_modified variable. Due to the u->headers_in.last_modified_time not being correctly initialized, this variable was evaluated to "Thu, 01 Jan 1970 00:00:00 GMT" for responses cached without the "Last-Modified" header which resulted in subsequent proxy requests being sent with "If-Modified-Since: Thu, 01 Jan 1970 00:00:00 GMT" header. Signed-off-by: Piotr Sikora <piotr@cloudflare.com>