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).
The string in that test was previously considered "not HTML" because it has a `#` before the first `<`. @davidchambers this is starting to feel prickly enough to warrent its own issue, since this pull request currently just moves an existing function to a separate file. How would you feel about following up with this separately? I think we could use a dedicated thread to decide the most efficient and faithful behavior.