CloudSigma provider ex_list_subscriptions() fails if your subscription includes free tier resources

When using driver.ex_list_subscriptions(status='all', resources=None) to list your resources, you'll receive the error:

  File "/sw/libcloud/2.4.0/lib/python3.5/site-packages/libcloud/utils/iso8601.py", line 117, in parse_date
    m = ISO8601_REGEX.match(datestring)
TypeError: expected string or bytes-like object

CloudSigma gave some free resources to their customers a few years ago.

<CloudSigmaSubscription id=free_mem, resource=mem, amount=1073741824, period=None, object_uuid=None>
<CloudSigmaSubscription id=free_dssd, resource=dssd, amount=53687091200, period=None, object_uuid=None>

I suspect these are the problem resources with no end time.

I've added to iso8601.py on lines 117 and 118:

    if datestring is None:
        datestring = "9999-01-01T12:00:00+00:00"

Not sure if this is an appropriate fix but seems to fix it.