future strings do not work with urllib.urlencode

In Python 2.7.12 the following snippet throws an exception (KeyError: 47) in standard library `urllib`:

```
from future.types import newstr as str
import urllib
urllib.urlencode({"a": str("/foo")}, doseq=True)
```

This might be a bug with urllib itself, since the exception only appears with the `doseq` parameter.  However I was wondering whether this is something you might be able to fix.
