unit test: also test for memcache value deletion

test/unit/45.cfg

@@ -40,9 +40,8 @@ route{
 	$mct(test) = "1";
 	$mct(test) = 5;
 	xlog("stored value is $mct(test)");
-# not work at the moment?
-#	$mct(test) = null;
-#	xlog("stored value is $mct(test)");
+	$mct(test) = $null;
+	xlog("stored value is $mct(test)");
 
 	$mct(cnt) = 1;
 	$mcinc(cnt) = 1; # increment by 1