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).
We could replace `str.length >= 3` with `str !== '<>'`.