Fixed the issue of takeLast(items, 0)

Hi,

`takeLast` #85 in RxJava will throw an exception when count == 0, as `LinkedBlockingDeque` rejects count <= 0. However, in c#, TakeLast with 0 count is valid. The following codes is OK in C# (no exception and do nothing):
<pre>
            var source = Observable.Return(1).TakeLast(0);
            source.Subscribe(
                x => 
                    Console.WriteLine("subscriber got " + x)
            );
            Console.ReadLine();
</pre>

I fixed this issue in this pull request. Please take a look. Thanks.