Fix a VCL expression type error: TIME +/- DURATION should return TIME

Fixes	#1211

Thanks to:	Federico G. Schwindt

bin/varnishtest/tests/r01211.vtc

+varnishtest "TIME +/- DURATION -> TIME"
+
+server s1 {
+	rxreq
+	txresp
+} -start
+
+varnish v1 -vcl+backend {
+
+	sub vcl_deliver {
+		set resp.http.bar = now + 4 d;
+		set resp.http.foo = now - 4 d;
+	}
+
+} -start
+
+client c1 {
+	txreq
+	rxresp
+	expect resp.http.bar ~ "..., .. ... 20[1-9][0-9] ..:..:.. GMT"
+	expect resp.http.foo ~ "..., .. ... 20[1-9][0-9] ..:..:.. GMT"
+} -run

lib/libvcl/vcc_expr.c

@@ -847,6 +847,9 @@ vcc_expr_add(struct vcc *tl, struct expr **e, enum var_type fmt)
 		ERRCHK(tl);
 		if (tk->tok == '-' && (*e)->fmt == TIME && e2->fmt == TIME) {
 			/* OK */
+		} else if ((*e)->fmt == TIME && e2->fmt == DURATION) {
+			f2 = TIME;
+			/* OK */
 		} else if (tk->tok == '-' &&
 		    (*e)->fmt == BYTES && e2->fmt == BYTES) {
 			/* OK */