context should allow use of DocumentFragment objects

Problem/Motivation

When adding new items to the DOM, its possible to add multiple adjacent elements as part of a DocumentFragment. This allows adding to a list of items, for example, without complicating DOM sibling traversal with "previousElementSibling", et al.

However, the once function does not allow the use of DocumentFragments as the "context" from which to select elements.

Steps to reproduce

1. Create a new DocumentFragment()
2. Attempt to select items with the new fragment as the context
3. Get new type error: "The element must be an instance of Element"

Proposed resolution

We don't actually want to check if context is an element. Instead we can check one of two things:

1. if the item is an instance of Document, DocumentFragment, or Element. (classes with a querySelectorAll method)
2. if the typeof item.querySelectorAll is a function

Remaining tasks

- Select a solution
- Do it

User interface changes

None

API changes

The API would allow more data types to be used as "context" without impacting backwards compatibility

Data model changes

None