Fixed validation for Members API newsletters endpoint

refs https://github.com/TryGhost/Toolbox/issues/465
refs 31546a6fd3/packages/admin-api-schema/lib/schemas/members.json (L93-L103)

- Updated Members API newsletter validation to match the subset of Admin API's members validation schema.

ghost/core/test/e2e-frontend/members.test.js

@@ -187,6 +187,8 @@ describe('Front-end members behavior', function () {
             getJsonResponse.newsletters.should.have.length(1);
 
             // Can update newsletter subscription
+            const originalNewsletters = getJsonResponse.newsletters;
+
             const res = await request.put(`/members/api/member/newsletters?uuid=${memberUUID}`)
                 .send({
                     newsletters: []
@@ -198,6 +200,18 @@ describe('Front-end members behavior', function () {
             jsonResponse.should.have.properties(['email', 'uuid', 'status', 'name', 'newsletters']);
             jsonResponse.should.not.have.property('id');
             jsonResponse.newsletters.should.have.length(0);
+
+            const resRestored = await request.put(`/members/api/member/newsletters?uuid=${memberUUID}`)
+                .send({
+                    newsletters: originalNewsletters
+                })
+                .expect(200);
+
+            const restoreJsonResponse = resRestored.body;
+            should.exist(restoreJsonResponse);
+            restoreJsonResponse.should.have.properties(['email', 'uuid', 'status', 'name', 'newsletters']);
+            restoreJsonResponse.should.not.have.property('id');
+            restoreJsonResponse.newsletters.should.have.length(1);
         });
 
         it('should serve theme 404 on members endpoint', async function () {

ghost/members-api/lib/repositories/member.js

@@ -405,6 +405,12 @@ module.exports = class MemberRepository {
             'expertise'
         ]);
 
+        if (data.newsletters) {
+            data.newsletters = data.newsletters.map(newsletter => ({
+                id: newsletter.id
+            }));
+        }
+
         // Trim whitespaces from expertise
         if (memberData.expertise) {
             memberData.expertise = memberData.expertise.trim();