astype(unicode) does not work as expected
astype unicode seems to call str, so that the following code throws

```
import pandas
df = pandas.DataFrame({"somecol": [u"適当"]})
df["somecol"].astype("unicode")
```

raises :

```
UnicodeEncodeError: 'ascii' codec can't encode ch
aracters in position 0-1: ordinal not in range(12
8)
```
