Fix `list:true` when referencing a cached schema

When using the `list: true` option, if the schema has already been cached in `Validator.schemas`, json-schema would not wrap the schema in `{ "type": "array", "items": ... }` as expected.

Test included to demonstrate the failure; the fix was to use `fake_uri` as is done elsewhere to generate an identifier for the wrapper schema. I didn't add a test to the `test_jsonschema_draft3` file, as this appears to be an identical code path, so the duplication shouldn't be necessary AFAICT.