Issue: Core Authorization fails when adding Roles to User

[jakbutler]

Description

The new Core Authorization support is sending a list of Role objects when assigning roles to a User via the UserEntity. The API is expecting a list of Strings.

Prerequisites

• I have checked the documentation for this library in the README.
• I have checked the Auth0 Community for related posts.
• I have checked for related or duplicate Issues and PRs.
• I have read the Auth0 general contribution guidelines.
• I have read the Auth0 Code of Conduct.
• I am reporting this to the correct repository [Add links to other related].

Environment

• Version of this library used: 1.13.0
• Version of Java used: jdk1.8.0_102
• Additional libraries that might be affecting your instance: N/A

Reproduction

Calling the addRoles or removeRoles endpoints via the UsersEntity, as below, returns a 400 error:

// in this example, role is of type com.auth0.json.mgmt.Role
getManagementAPI().users().addRoles("userId", Collections.singletonList(role)).execute()

{
  "statusCode": 400,
  "error": "Bad Request",
  "message": "Payload validation error: \u0027Expected type string but found type object\u0027 on property roles[0].",
  "errorCode": "invalid_body"
}