unicode encode error on index joins
The following throws a `UnicodeDecodeError`.

```
In [8]: df = mkdf(10, 10, data_gen_f=lambda r,c:randn(), r_idx_type='dt', c_idx_type='u')

In [9]: s = Series(randn(5,), df.index[:5])

In [10]: s.index.join(df.columns, how='outer').join(s.index)
```
