Fix incorrect representation when using reals

Fixes #1532

bin/varnishtest/tests/r01532.vtc

+varnishtest "Incorrect representation when using reals - #1532"
+
+server s1 {
+	rxreq
+	txresp
+} -start
+
+varnish v1 -vcl+backend {
+	import ${vmod_std};
+
+	sub vcl_deliver {
+		set resp.http.x-foo = std.real2time(1140618699.00);
+	}
+} -start
+
+client c1 {
+	txreq
+	rxresp
+	expect resp.http.x-foo == "Wed, 22 Feb 2006 14:31:39 GMT"
+} -run

lib/libvcc/vcc_expr.c

@@ -749,7 +749,7 @@ vcc_expr4(struct vcc *tl, struct expr **e, enum var_type fmt)
 			e1 = vcc_mk_expr(BYTES, "%.1f", d);
 			ERRCHK(tl);
 		} else if (fmt == REAL) {
-			e1 = vcc_mk_expr(REAL, "%g", vcc_DoubleVal(tl));
+			e1 = vcc_mk_expr(REAL, "%f", vcc_DoubleVal(tl));
 			ERRCHK(tl);
 		} else {
 			e1 = vcc_mk_expr(INT, "%.*s", PF(tl->t));