Unexpected difference between loc and ix
See my [StackOverflow post](http://stackoverflow.com/questions/34259637/unexpected-difference-between-loc-and-ix) for details.  I've come to the conclusion that this is a bug -

```
import pandas as pd

# Create a dataframe
df = pd.DataFrame({'id':[10,9,5,6,8], 'x1':[10.0,12.3,13.4,11.9,7.6], 'x2':['a','a','b','c','c']})
df.set_index('id', inplace=True)
df.loc[[10, 9, 7]] # 7 does not exist in the index so a NaN row is returned
df.loc[[7]] # KeyError: 'None of [[7]] are in the [index]'
df.ix[[7]] # 7 does not exist in the index so a NaN row is returned
```

I think `df.loc[[7]]` should not return an error, but should return a dataframe with 1 row for id=7 with NaN values.
