Correct bug in implementation of `Cheerio#val`

jQuery does not attempt to calculate the value of a `radio` input group.
Instead, when the value of such an element is queries, that element's
`value` attribute is returned directly.

Although slightly less convenient in some cases, this allows for direct
manipulation of `radio` input element values (which is otherwise
impossible).
Merge pull request #544 from jugglinmike/fix-val

Correct bug in implementation of `Cheerio#val`