NameError: name 'required_headers' is not defined

```
"drf_httpsig/authentication.py", line 50, in authenticate_header
    h = " ".join(required_headers)
NameError: name 'required_headers' is not defined
```

This happens only if drf-httpsig is the primary auth method for DRF.

May be code should be `self.required_headers` 