Null DatetimeField raises an exception upon serialisation request

[system7ltd]

Objects with DatetimeField attributes cannot be properly serialised when any of their DatetimeField attributes is not set (i.e. set to null in the JSON response). This makes the following scenario possible:

1. fetch data from the server (e.g. TembaClient(...).get_runs()) - the data is internally deserialised by rapidpro-python
2. fed the result of this deserialisation process back to rapidpro-python to have it (re)serialised
3. error (please see the example traceback below)

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/home/ego/.virtualenvs/rapidpro-python/local/lib/python2.7/site-packages/temba_client/serialization.py", line 63, in serialize
    field_value = field.serialize(attr_value)
  File "/home/ego/.virtualenvs/rapidpro-python/local/lib/python2.7/site-packages/temba_client/serialization.py", line 148, in serialize
    return [self.item_class.serialize(item) for item in value]
  File "/home/ego/.virtualenvs/rapidpro-python/local/lib/python2.7/site-packages/temba_client/serialization.py", line 63, in serialize
    field_value = field.serialize(attr_value)
  File "/home/ego/.virtualenvs/rapidpro-python/local/lib/python2.7/site-packages/temba_client/serialization.py", line 122, in serialize
    return format_iso8601(value)
  File "/home/ego/.virtualenvs/rapidpro-python/local/lib/python2.7/site-packages/temba_client/utils.py", line 40, in format_iso8601
    return six.text_type(value.astimezone(pytz.UTC).strftime(_format))
AttributeError: 'NoneType' object has no attribute 'astimezone'

[rowanseymour]

PR looks fine besides small tweak. Am wondering what you are using serialization for?