Don't envelop fragments in documents.

Prevents a body tag from being matched in an empty document.

This changes the expectations for `document_root_element` so we need to update Rails as well. I'll submit a separate PR once this is merged.

I'm pretty tired of issues like this because it's so hard to figure out if the node being searched from is included in the search or not. We're probably going to have more issues because of that behavior :cry:

Closes #19