can't filter collections by tags or categories

Specifying both a collection and tag / category to use for filtering it in the front matter:

```
paginate:
  collection: my_collection
  tag: parts
```

results in an undefined method error for `Jekyll::Document`

So looks like tags assigned to collection items aren't accessible in the same way as tags assigned to posts? (happening in the reject loop of `def collection(page)`)
