Fixed a bug in removing type="hidden" attr

If you try to remove the type attribute from inputs when it's set to `hidden` Cheerio incorrectly assumes that it's boolean.

If I understand what `rboolean` is used for then this should fix this bug and prevent future similar ones.

Here's a failed test with the current version of Cheerio:

    var cheerio = require("cheerio"),
    	$ = cheerio.load('<input type="hidden" />');

    $("input").removeAttr("type"); // Type is now set to false instead of being removed

    var out = $.html(); // This will fail since it thinks type is a string