This needs to use `unicode()` instead of `str`. 
Imagine if you have...
```
api = models.FooBar()
api.get(signatures=[u'Bärs', u'Båge', u'Börs'])
```
Then that becomes `u'B\xe4rs+B\xe5ge+B\xf6rs'` first which is later turned into a byte string inside `urlencode_params()`