Clone input nodes when inserting over a set

I noticed this bug while *wrapping* up gh-558 (c'mon that's funny). I think the changeset ended up being a little ugly, so please don't hesitate to offer suggestions for refactoring!

Commit message:

> This patch increases parity with jQuery, specifically in cases where
> manipulation methods are called with existing nodes on sets containing
> more than one element. In such cases, the provided node should be cloned
> prior to insertion (excepting the final element in the set, where the
> original node should be inserted).
Okay. We can remove the `#` in that case, giving:

```javascript
return /^[^<]*<[\s\S]+>[^>]*$/.test(str);
```

Also, since we're no longer using capturing groups, matching the entire input is unnecessary. We can remove the `^` and `$` along with the `[^<]*` and `[^>]*`:

```javascript
return /<[\s\S]+>/.test(str);
```

*Now* we have something sensible!