Round down synthesized last-modified timestamp in object

Commit 3bd239e9 makes it so we set the
object's last-modified timestamp to the current timestamp if there's
no Last-Modified header.  This makes it so we can do conditional gets
even when there's no Last-Modified or ETag header from the backend.
However, the time entered was a float, so we effectively ended up with
an off-by-up-to-one-minus-epsilon error.  This lead to requests ending
up in a 304/200 response code round-dance.

We now round the timestamp down to the nearest whole second instead,
which should fix this bug.  Thanks to lrowe for a helpful bug report.

Fixes: #907

bin/varnishd/cache_center.c

@@ -785,7 +785,7 @@ cnt_fetchbody(struct sess *sp)
 	if (http_GetHdr(hp, H_Last_Modified, &b))
 		sp->obj->last_modified = TIM_parse(b);
 	else
-		sp->obj->last_modified = sp->wrk->entered;
+		sp->obj->last_modified = floor(sp->wrk->entered);
 
 	assert(WRW_IsReleased(sp->wrk));
 

bin/varnishtest/tests/r00907.vtc

+varnishtest "Ticket #907 200/304 handling with Etags + Last-Modified"
+
+server s1 {
+	rxreq 
+	txresp \
+	       -hdr "ETag: saengei1Ohshicich4iteesu"
+} -start
+
+varnish v1 -vcl+backend {
+	sub vcl_deliver {
+		set resp.http.x-timestamp = now;
+	}
+} -start 
+
+client c1 {
+	txreq -hdr "If-None-Match: saengei1Ohshicich4iteesu"
+	rxresp
+	expect resp.status == 304
+	txreq -hdr "If-None-Match: saengei1Ohshicich4iteesu" \
+	      -hdr "If-Modified-Since: ${date}"
+	rxresp
+	expect resp.status == 304
+} -run