fixed NibCell doesn't work well

the height method won't work for nibCell
I like that this supports both selectors now, but please maintain the original order of the API.

```objc
if ([object respondsToSelector:@selector(collectionViewCellClass)]) {
  // ...
}
if (nil == cell && [object respondsToSelector:@selector(collectionViewCellNib)]) {
}
```