Fix `Cannot convert object to primitive value`
Fix `Cannot convert object to primitive value`

If `root` is created from `Object.create(null)` then `String(root)` throws `Cannot convert object to primitive value` which results in the confusing error `Failed prop type: Cannot convert object to primitive value` instead of the expected error `Failed prop type: currentUser missing on {}`
apollostack/apollo-client,apollographql/apollo-client,apollostack/apollo-client,apollostack/apollo-client,apollographql/apollo-client