examine that Buffer() implementation follows Window() behavior

[atsushieno]

"A tale about Buffer" is worth following and examining if our implementation is correct (we don't have to reimplement Buffer() since ours is already different; performance could be reviewed but it does not apply only to this). http://blogs.msdn.com/b/rxteam/archive/2012/03/12/reactive-extensions-v2-0-beta-available-now.aspx