[Python] None value in "object" Swagger type are incorrectly converted to "None" and not null

[lmazuel]

In [1]: import msrest.serialization
In [2]: serializer = msrest.serialization.Serializer()
In [3]: serializer.serialize_object({'test': None})
Out[3]: {'test': 'None'}

this should be None, then the JSON serializer can translate as the correct "null" in the JSON payload

Expected:

In [1]: import msrest.serialization
In [2]: serializer = msrest.serialization.Serializer()
In [3]: serializer.serialize_object({'test': None})
Out[3]: {'test': None}