Closes issue #33.

```
self = <test_authentication.TestAuthenticateMessage testMethod=test_authenticate_dmarc_prev_spf>

    def test_authenticate_dmarc_prev_spf(self):
        prev = "Authentication-Results: example.com; spf=pass smtp.mailfrom=gmail.com"
>       res = authenticate_message(self.message2, "example.com", prev=prev, spf=False, dkim=True, dmarc=True, dnsfunc=self.dnsfunc)
              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

authheaders/test/test_authentication.py:155:
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _
authheaders/__init__.py:434: in authenticate_message
    return str(auth_res)
           ^^^^^^^^^^^^^
/usr/lib/python3/dist-packages/authres/core.py:476: in __str__
    return ''.join((self.HEADER_FIELD_NAME, ': ', self.header_value()))
                                                  ^^^^^^^^^^^^^^^^^^^
/usr/lib/python3/dist-packages/authres/core.py:492: in header_value
    strs.append(str(result))
                ^^^^^^^^^^^
/usr/lib/python3/dist-packages/authres/core.py:222: in __str__
    strs.append(str(property_))
                ^^^^^^^^^^^^^^
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _

self = <authres.core.AuthenticationResultProperty object at 0x7fc4178465f0>

    def __str__(self):
        if self.comment:
            return '%s.%s=%s (%s)' % (self.type, self.name, self.value.quote_if_needed(), self.comment)
        else:
>           return '%s.%s=%s' % (self.type, self.name, self.value.quote_if_needed())
                                                       ^^^^^^^^^^^^^^^^^^^^^^^^^^
E           AttributeError: 'NoneType' object has no attribute 'quote_if_needed'

/usr/lib/python3/dist-packages/authres/core.py:132: AttributeError
```