No change notification for props beginning with underscore with Stateful#observe().

From: https://github.com/asudoh/decor/commit/1679d7657946e66cd835a62c378b536d357ceb55#commitcomment-7057420
The original code was for filtering out changes in shadow properties which won't happen unless native Object.observe() is used. The code is changed to filter out all underscore properties which may happen even with shim if application sets underscore property via Observable#set().

Now... I guess I talked to my rubber-duck which makes me realize that underscore property not setting up shadow property does not mean application cannot emit change record with it. So I guess it has become true again filtering out underscore variable no matter what.