Don't copy a property which is not a plain object but an object. 

By modification to fix #3, a following bug is included:

```js
function O(v) {
  this.a = { b: { c: v } };
}
var o1 = new O(123);
var o2 = new O(456);
var p1 = { o: o1 };
var p2 = { o: o2 };
copyProps(p1, p2);
console.log(p2.o === o1); // => false, but it should be true!
```

This bug can be fixed by using `isPlainObject` to check an empty plain object instead of `isObject`.