$.map is to eager on testing null

[mikael@…]

Given

console.log($.map([ 1, undefined, , 2, null, 3 ], function (e, i) { return e; }));

results in

[ 1, 2, 3 ]

where I'd expect

[ 1, undefined, undefined, 2, 3 ]

Underlying problem is that we compare using value != null instead of value !== null

[dmethvin]

I am sure there is code depending on undefined to remove the element by now, so I don't think we want to change the behavior. Instead I think we should document the current behavior.

[dmethvin]

Docs updated.