jQuery.extend demonstrates unexpected behavior. Fields from objectN with value of ''undefined'' does not copy to target object.

Example:

var target = {};
var object1 = {
    undefined_field: undefined, 
    null_field: null
};
var result = $.extend({}, target, object1);

console.log('target:');
for (var field in target)
    console.log(field, target[field]);
console.log('');

console.log('object1:');
for (var field in object1)
    console.log(field, object1[field]);
console.log('');

console.log('result:');
for (var field in result)
    console.log(field, result[field]);

Output:

> target:
> 
> object1:
> undefined_field undefined
> null_field null
> 
> result:
> null_field null

Expected output:

> target:
> 
> object1:
> undefined_field undefined
> null_field null
> 
> result:
> undefined_field undefined
> null_field null

Bug reproduces with jQuery versions of 1.8.3, 1.10.2, 2.0.3.