Immutable.Stack#last() returns undefined

**Version:**
3.6.2

**Bug:**
Stack should return the last value when `last()` is called.

**Example:**

``` javascript
var l = Immutable.List([1, 2, 3, 4]);
var s = Immutable.Stack(l);
s.first(); // 1
s.last(); // undefined
s.size; // 4
s.get(3); // 4
```
