combine_first behavior change

``` python
from pandas import DataFrame
from datetime import datetime
df = DataFrame({'US': [1]}, index=[datetime(2012,1,1)])
df2 = DataFrame({}, columns=['EU'])
df.combine_first(df2)
```

In 0.9.0, this returns:

```
            US
2012-01-01   1
```

In 0.10.0b1, this returns:

```
            EU  US
2012-01-01 NaN   1
```
