Representation data that's a string, but with members doesn't get recognized as an enum

[erunion]

* @api-data showing (string) - Whether the values of the upload_quota.space fields 
*      are for the lifetime quota or the periodic quota.
*      + Members
*          - `periodic`
*          - `lifetime`

When compiling this, Mill sees it as just a string and ignores the supplied enums. Ideally, this data would be written as enum instead of string, but Mill should be able to detect that it's a string+enums.

// @kenyavs

[erunion]

Fixed in v4