Inconsistent NA handling in comparison ops with dtype=object

```
test=PD.Series(index=PD.DateRange(DT.datetime(2012,3,15), DT.datetime(2012,3,16)), data=NP.ones(2))
test == test.shift(1)
```

vs

```
test2=PD.Series(index=PD.DateRange(DT.datetime(2012,3,15), DT.datetime(2012,3,16)), data=['a','a'])
test2 == test2.shift(1)
```
