allow_custom is not inherited to common properties

[EfstratiosLontzetidis]

The issue is that the allow_custom=True is not inherited in common properties that contain custom_properties.
Common STIX properties allow for adding custom properties using the allow_custom=True option. So, STIX does not restrict us for adding custom properties in a Common STIX property.

An example to reproduce the issue is:

1. Create an "External Reference" common property and add some custom properties
2. Create a Malware SDO and add the external-reference of the previous step.
3. Save the Malware SDO to a json file
4. Try to parse this Malware SDO via obj = parse(file_handle, allow_custom=True)

The error that is returned is the following:
"stix2.exceptions.InvalidValueError: Invalid value for Malware 'external_references': Unexpected properties for ExternalReference: (customizedproperty)."

This might be related to the issue #176 and issue #333.
I apologize if it is a false positive and I am doing something wrong. Please let me know what I should do.

The json file is the following:

    "type": "malware",
    "spec_version": "2.1",
    "id": "malware--83687fef-12ec-4959-94fe-a4f1c1380747",
    "created": "2021-03-06T18:54:28.533Z",
    "modified": "2021-03-06T18:54:28.533Z",
    "name": "malware-test-inheritance-of-custom-property",
    "description": "test-inheritance-of-custom-property",
    "is_family": false,
    "first_seen": "2021-03-06T18:54:28.533Z",
    "external_references": [
        {
            "source_name": "test-inheritance-custom",
            "description": "test for inheritance",
            "customizedproperty": "acustomproperty"
        }
    ]
}

[clenk]

This should now be fixed in #503.