switch to is instead of equals for None comparison
switch to is instead of equals for None comparison

weikang9009/pysal,lixun910/pysal,sjsrey/pysal,ljwolf/pysal,ljwolf/pysal,pysal/pysal,pedrovma/pysal,lanselin/pysal,lixun910/pysal,ljwolf/pysal,lixun910/pysal