Fix: prevent global option from changing when merging

Author: arrowrowe

lib/prepare.js

@@ -26,7 +26,7 @@ module.exports = function (assetsOrigin) {
 
     var pkg = pkgs[pkgName];
     pkg.name = pkgName;
-    var option = util.merge(optionGlobal, util.extend({
+    var option = util.merge(clone(optionGlobal), util.extend({
       mode: ['copy', -1],
       hash: [0, -1],
       export: [true, -1]