.attr with value of undefined returns undefined

[iwarren]

When calling attr and passing in undefined as the value of the attr you are trying to set cheerio will return undefined. This is different than jQuery:

Assume $ = jQuery

$('<div/>').attr('id', 'foo').attr('id')
=> 'foo'
$('<div/>').attr('id', null).attr('id')
=> undefined
$('<div/>').attr('id', undefined)
=> $('<div/>')
$('<div/>').attr('id', undefined).attr('id')
=> undefined

No problems. Now, assume $ = cheerio

$('<div/>').attr('id', 'foo').attr('id')
=> 'foo'
$('<div/>').attr('id', null).attr('id')
=> undefined
$('<div/>').attr('id', undefined)
=> undefined
$('<div/>').attr('id', undefined).attr('id')
=> TypeError: Cannot call method 'attr' of undefined

I have forked cheerio and created a failing test on master that shows this behavior (iwarren@bc75efb). I did not make the corresponding fix as I wasn't sure if this was an intentional divergence from jquery.