attr converts undefined to string

[fb55]

jQuery removes attributes when setting their value to undefined, while cheerio sets them to the string 'undefined'.

Discovered in #1659.

[5saviahv]

Notes:

in jQuery if value is specified, but its value is undefined it returns this object

const ret = elems.attr('name', undefined)
// ret == elems

if value is not specified, it tries get first element attribute value, if it is not existing it returns undefined

const ret = elems.attr('name')
// ret == undefined