AttributeError: 'str' object has no attribute 'quote_if_needed'

This seems to be a core bug, as we don't check if `value` (Str) has the attribute `quote_if_needed` (Function).

Full stack trace is provided below:

```sh
Error: Traceback (most recent call last):
  File "/var/www/production/source/node_modules/authheaders/scripts/authenticate-message.py", line 36, in <module>
    main()
  File "/var/www/production/source/node_modules/authheaders/scripts/authenticate-message.py", line 25, in main
    header = authheaders.authenticate_message(msg=message, authserv_id=authservId, ip=ip, mail_from=mailFrom, helo=helo, spf=True, dkim=True, arc=True)
  File "/home/deploy/.local/lib/python3.6/site-packages/authheaders/__init__.py", line 298, in authenticate_message
    return str(auth_res)
  File "/home/deploy/.local/lib/python3.6/site-packages/authres/core.py", line 476, in __str__
    return ''.join((self.HEADER_FIELD_NAME, ': ', self.header_value()))
  File "/home/deploy/.local/lib/python3.6/site-packages/authres/core.py", line 492, in header_value
    strs.append(str(result))
  File "/home/deploy/.local/lib/python3.6/site-packages/authres/core.py", line 222, in __str__
    strs.append(str(property_))
  File "/home/deploy/.local/lib/python3.6/site-packages/authres/core.py", line 132, in __str__
    return '%s.%s=%s' % (self.type, self.name, self.value.quote_if_needed())
AttributeError: 'str' object has no attribute 'quote_if_needed'
```