Not all parameters are strings, _utf8 assumes that they all are

If I call:
```
  link_clicks(link="http://bitly.com/Wozuff", rollup=False, unit="hour", units=-1)
```
with an integer units parameter (as noted in your docs), it throws:
```
...
bitly_api/bitly_api.pyc in _utf8(s)
...
---> 24     assert isinstance(s, str)
```

If you provide a string for units then 
```
_call_oauth2_metrics(self, endpoint, params, unit, units, tz_offset, rollup, limit)
...
--> 632   assert isinstance(units, int), "Unit (%r) must be integer" % units

```
throws an error.

Why does _uft8 assume that every parameter is a string?