Unicode issue with HDFS

When running , I faced Unicode issues:
odo(df_pd, 'hdfstore://F://data.h5::/data/')

In pandas, we can filter it and re-code it.

```python
types = df_pd.apply(lambda x: pd.lib.infer_dtype(df_pd.values))
types[types=='unicode']
for col in types[types=='unicode'].index:
df_pd[col] = df_pd[col].astype(str)
```
Is there a way to do it in Blaze ?

repost from: https://github.com/blaze/blaze/issues/1607 from @arita37