Cannot support list of dates in Email Message Header

[gtback]

Minimal example to reproduce:

import datetime as dt
from cybox.objects.email_message_object import EmailHeader
hdr = EmailHeader()
hdr.date = [dt.datetime.now(), dt.datetime.now()]
hdr.to_xml()

Returns:

~/code/mixbox/mixbox/dates.py in serialize_datetime(value)
     39     if not value:
     40         return None
---> 41     return value.isoformat()
     42 
     43 

AttributeError: 'list' object has no attribute 'isoformat'

I'm not sure the best place to fix this. It may be using cybox.properties.ListDateTimeField rather than cybox.properties.DateTime with a normal TypedField. But this change would probably need to be more pervasive than just this one field. It needs more investigation.

From STIXProject/python-stix#350