Basic Auth handler sets unicode header value on Python 2

Observe:

```python
>>> import requests
>>> requests.__version__
'2.3.0'
>>> r = requests.Request('GET', 'http://www.google.com/', auth=('user', 'pass'))
>>> p = r.prepare()
>>> p.headers['Authorization']
u'Basic dXNlcjpwYXNz'
```

This is wrong: all header keys and values should be native strings. Easily fixed though. =)