Add support to datetime for "date" CustomFields

[ehooo]

Feature Request

Add support on CustomJsonEncoder for datetime objects

Work Environment

Question	Answer
TheHive4py version / git hash	1.6.0

Problem Description

The JsonEncoder not support datetime objects used on CustomFields

Steps to Reproduce

1. Run the code:

custom_fields = CustomFieldHelper()
custom_fields.add_date('date', datetime.now())
case = Case(title='Example', description='description', customFields=custom_fields.build())
api.create_case(case)

Possible Solutions

Improve object CustomJsonEncoder allowing the datetime objects