Account for a cached value of False when validating.

[dmpayton]

Hi there,

I had an issue on django-flanker that I've traced to flanker itself.

dmpayton/django-flanker#2

If connect_to_mail_exchanger throws an exception or fails to connect for whatever, it returns None and mail_exchanger_lookup will set mx_cache[domain] to False.

This is not accounted for on subsequent calls to validate_address. If the cached value is None, it returns None, otherwise it continues. If the cached value is False, it throws the following exception:

Traceback (most recent call last):
  File "<console>", line 1, in <module>
  File "/home/derek/dev/django-flanker/flanker/utils.py", line 99, in wrapper
    return_value = f(*args, **kwargs)
  File "/home/derek/dev/django-flanker/flanker/addresslib/address.py", line 205, in validate_address
    plugin = flanker.addresslib.validate.plugin_for_esp(exchanger)
  File "/home/derek/dev/django-flanker/flanker/addresslib/validate.py", line 108, in plugin_for_esp
    if grammar[0].match(mail_exchanger):
TypeError: expected string or buffer

This patch simply returns None if the cached value is False.