Use `code.set` rather than `insert.set`.

Hello Tyler,

This change addresses some issues we're seeing where Redactor treats the incoming html as user input from a human, rather than code input from the backend.

* http://imperavi.com/redactor/docs/api/code/#api-set
* http://imperavi.com/redactor/docs/api/insert/#api-set

In our case, `insert.set` collapses a sequence of `<p><br></p>` tags even if the user entered and desires them, while `code.set` leaves them intact.

Would this change make sense for the larger community as well?

Thanks!

- Brandon