function keys (obj) {
    var keysArray = [];
    for (var k in obj) {
        if (hasOwn.call(obj, k)) {
            keysArray.push(k);
        }
    }
    return keysArray;
}
Returns keys from an Object as a Stream.

@id keys
@section Objects
@name _.keys(obj)
@param {Object} obj - the object to return keys from
@api public

_.keys({foo: 1, bar: 2, baz: 3})  // => 'foo', 'bar', 'baz'