assert_almost_equal fails with KeyError on non-integer column names
```
import pandas as pd

df = pd.DataFrame(0:'b', index = ['c'])
pd.util.testing.assert_almost_equal(df, df)  # SUCCESS

df = pd.DataFrame({'a':'b'}, index = ['c'])
pd.util.testing.assert_almost_equal(df, df)  # FAIL KeyError
```

Top of the trace stack is:
- pandas\hashtable.pyd in pandas.hashtable.PyObjectHashTable.get_item (pandas\hashtable.c:12310)()
- pandas\hashtable.pyd in pandas.hashtable.PyObjectHashTable.get_item (pandas\hashtable.c:12261)()
- KeyError: 0

I am sure this used to work without any problem, because I have unit tests that depend on it.

I tried uninstalling and re-installing pandas and the bug is still present.

I couldn't find a similar bug searching for get_item, assert_almost_equal or hashtable.
