Invalid thing description for nullable fields

[rwb27]

Describe the bug If any properties/actions have schemas where any of the fields have allow_none=True, the thing description is invalid, because it will contain a schema that looks like:

{"type": ["string", "null"]}

Lists are valid in JSONSchema, but are not permitted in Thing Description DataSchema objects.

Expected behaviour List types should be converted to multiple schemas, and combined using oneOf. This is valid in both JSONSchema and Thing Description. For example, fields.String(allow_none=True) should render as:

{
    "oneOf": [
        {"type": "string"},
        {"type": "null"}
    ]
}