BUG: ensure index casting works even in Int64Index

```
df = DataFrame({'A' : [1.1,2.2,3.3], 'B' : [5.0,6.1,7.2]}, index = [2010, 2011, 2012])
df.index = df.index.astype(int)
df.ix[2010]
```

was failing - fixed up

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