[json] improve property suggestions with oneOf

<!-- Do you have a question? Please ask it on https://stackoverflow.com/questions/tagged/vscode. --> <!-- Use Help > Report Issue to prefill these. --> - VSCode Version: Version 1.20.0-insider - OS Version: 10.13.3 Steps to Reproduce: 1. Use this JSON schema ```json { "$schema": "http://json-schema.org/draft-04/schema#", "type": "object", "oneOf": [ { "title": "Wrapper", "type": "object", "required": [ "asset" ], "properties": { "asset": { "type": "object" } } }, { "title": "No Wrapper", "type": "object", "properties": { "id": { "type": "string" } } } ] } ``` 2. Start writing JSON using this schema and type: ```json { "" } ``` 3. Only id is suggested as an option, I expect both asset and id to be suggestions NOTE: if i make "id" also required I get the correct suggestions. <!-- Launch with `code --disable-extensions` to check. --> Does this issue occur when all extensions are disabled?: Yes/No Yes