I have this little utility/helper which serializes a form into a javaScript object

            $.fn.serializeObject = function(fn) {
              var o = {},
                  a = this.serializeArray(),
                  that;
              $.each(a, function() {
                that = this;
                  if($.isFunction(fn)){
                    that = fn(that.name, that.value);
                  }
                if(o[this.name] !== undefined) {
                  val = that.value || '';
                  if(!o[that.name].push) {
                    o[that.name] = [o[that.name]];
                  }
                  o[that.name].push(that.value);
                } else {
                  o[that.name] = that.value;
                }
              });
              return o;
            };

A typical usage of this plugin will be:

            someNameSpace = {
                //... 
                processForm: function (form) {
                    return JSON.stringify(form.serializeObject(function (key, val) {
                        return {
                            name: key,
                            value: encodeURIComponent(val)
                        };
                    }));
                }
                //...
            };

Any suggestions on how to improve this piece of code?