I *think* from my quick look that you might have an alternative here that's a bit more general even. If you make this thing take a `request` method off a session, then you'd get what you want, plus also people could configure `requests.Session`s as they wish.

E.g.,

```
session = requests.Session()
session.auth = "username", "password"
session.headers.update(Stuff="1234")
solr = Solr(make_request=session.get)
```

(where I also think you could then deprecate the `auth` parameter here).