unexpected join behavior for datetime indexes with integers
I can't tell if this is bug or not. I think object indices should stay as object indices if they are originally mixed, but currently an index join converts something date-ish to dates...

```
In [9]: df = mkdf(10, 10, data_gen_f=lambda*args:randn(), r_idx_type='i', c_idx_type='dt')

In [10]: df.columns.join(df.index, how='outer').join(df.columns)
Out[10]:
<class 'pandas.tseries.index.DatetimeIndex'>
[2000-01-03 00:00:00, ..., 1970-01-01 00:00:00.000000009]
Length: 20, Freq: None, Timezone: None
```

this could be an edge case, but it's a problem when trying to join indexes in a general way for upcoming `pandas.eval`...i.e., i can't just call `align`.
