Use `he` for robust HTML entity encoding/decoding
Change test to reflect `he`'s encoding of `<`

Looks like the `<`-sign is encoded differently by [ent] and [he][2].

[1]: https://github.com/substack/node-ent
[2]: https://github.com/mathiasbynens/he
Merge branch 'use-he'

Closes #1.