Redo weak equality check so we can colorize null in safe mode (#257)

Author: givehug

lib/colors.js

@@ -105,7 +105,8 @@ function applyStyle() {
   var args = Array.prototype.slice.call(arguments);
 
   var str = args.map(function(arg) {
-    if (arg !== undefined && arg.constructor === String) {
+    // Use weak equality check so we can colorize null in safe mode
+    if (arg != undefined && arg.constructor === String) {
       return arg;
     } else {
       return util.inspect(arg);