.ix() failure for mixed-integer index
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```
With the following DataFrame:

df = pandas.DataFrame(index=[1, 10, 'C', 'E'], columns=[1, 2, 3])

df.ix[df.index[:-1]] works
df.ix[:-1] doesn't

df.ix[[1, 10]] does not work
df.ix[pandas.Index([1, 10], dtype=df.index.dtype)] works
```

```
df.ix[10] does not work
```
