Keys with "undefined" as value should not be overridden

[roddik]

Values, which are set, should be used even if the value is undefined - example

var extend = require('deep-extend');

var a = {
    b: 1,
    c: 2
};

var b = {
    b: undefined,
    c: false
};

console.log(extend(a, b));

Actual output { b: 1, c: false } expected { b: undefined, c: false }. This line should instead use if (key in obj)