Issue with JSON datetime Serialization #230

Certain CybOX fields that use xs:dateTime as their base type will throw "not JSON serializable" errors when trying to serialize python-cybox objects parsed in from CybOX XML documents to JSON, due to their being marshaled into python datetime objects in the bindings. E.g., https://github.com/CybOXProject/python-cybox/blob/master/cybox/bindings/cybox_core.py#L915-921.

A likely solution would be to check for such objects in to_dict() of cybox.Entity and if found, cast them to a string before passing them into the dictionary. This is essentially what python-stix does in its to_dict() methods.

This can be replicated via:

from datetime import datetime
from cybox.core import Action

a = Action()
a.timestamp = datetime.now()

print a.to_json()

This is related to: MAECProject/python-maec#66 and MAECProject/python-maec@0d844b6