BUG: to_hdf failing with integer columns and data_columns=True

```
In [39]: N = 1000

In [40]: df = DataFrame(np.random.randint(0,8,size=N*400).astype('uint8').reshape(N,-1),index=pd.date_range('20130206',periods=N,freq='ms'))

In [41]: df.to_hdf('test.h5','df',mode='w',format='table')

In [42]: df.to_hdf('test.h5','df',mode='w',format='table',data_columns=True)
AttributeError: 'numpy.int64' object has no attribute 'startswith'
```
