Remove decode in wrong place.

```
  File "[...]/lib/python3.3/site-packages/requests_oauthlib/oauth1_auth.py", line 70, in __call__
    r.url = to_native_str(r.url)
  File "[...]/lib/python3.3/site-packages/requests_oauthlib/oauth1_auth.py", line 14, in to_native_str
    return string.decode('utf-8')
AttributeError: 'str' object has no attribute 'decode'
```
Request object have 'url' attribute with 'str' type, not 'bytes' so
decode in this place is wrong.