Correct behavior of `Cheerio#parents`

When invoked on a top-level element, `parents` should return an empty
set.

Because a unit test for `Cheerio#parents` and the implementation of
`Cheerio#val` depends on the erroneous behavior, this code must also be
updated.
Merge pull request #245 from jugglinmike/parents-bug

Correct behavior of `Cheerio#parents`