use isArray or +length？

When I read the source file, I found that if i write code as follow, it will not correct.
```js
	var obj = {
		'heght': 2,
		'width': 3,
		'length': 4
	};
	var result = _.size(obj);
```
the size() function is:
```js
  // Return the number of elements in an object.
  _.size = function(obj) {
    if (obj == null) return 0;
    return (obj.length === +obj.length) ? obj.length : _.keys(obj).length;
  };
```

so, i think maybe this is a better way, isn't it?
```js
  // Return the number of elements in an object.
  _.size = function(obj) {
    if (obj == null) return 0;
    return _.isArray(obj) ? obj.length : _.keys(obj).length;
  };
```