Fix de/serialization of Invitation roles

[lbalmaceda]

Changes

The Invitation object has an array of strings for the roles ids. This is NOT an object and thus, should not be serialized wrapped as an object.

References

Fixes #378 without introducing breaking changes.

Testing

Manually tested with the following snippets. Also, fixed unit tests.

static void assignRole(ManagementAPI api) {
    try {
        api.organizations().addRoles(ORG_ID,
            USER_ID,
            new Roles(Arrays.asList(ADMIN_ROLE_ID)))
            .execute();
    } catch (Auth0Exception e) {
        e.printStackTrace();
    }
}

static void listInvitations(ManagementAPI api) {
    try {
        // Originally failed when invitation contains one or more Roles
        InvitationsPage page = api.organizations().getInvitations(ORG_ID, null).execute();
        for (Invitation i : page.getItems()) {
            System.out.println(i);
        }
    } catch (Auth0Exception e) {
        e.printStackTrace();
    }
}

static void sendInvitation(ManagementAPI api) {
    Invitation invitation = new Invitation(
        new Inviter("Luciano"),
        new Invitee("luciano.balmaceda@auth0.com"),
        CLIENT_ID
    );
    invitation.setRoles(new Roles(Arrays.asList(ADMIN_ROLE_ID)));
    Invitation result;
    try {
        // Originally failed because invitation contains one or more Roles
        result = api.organizations().createInvitation(ORG_ID, invitation).execute();
        System.out.println(result.getInvitationUrl());
    } catch (Auth0Exception e) {
        e.printStackTrace();
    }
}