Warn & restore boolean attribute treatment from 3.x

[mgol]

See jquery/api.jquery.com#1243 & jquery/jquery#5452 for changes planned for 4.0. We should restore special boolean attributes treatment and warn if the setter is used with an unrecognized value. In the getter, warn if the value is different than the lowercased attribute name.

Remember to go through the list of boolean attributes in the spec and exempt any new values (like until-found for the hidden attribute) from such a conversion. New attributes should not be added to the list but any changes in existing ones will need to be accounted for.

[mgol]

I have this work in progress but I'm pausing it for now to focus on other priorities; I'll return to this before the jQuery 4.0.0 RC.

[mgol]

PR: #540