Changed /email_previews/posts/<post id> endpoint to return 204 upon success

refs https://github.com/TryGhost/Toolbox/issues/308

- this endpoint is currently used to send a test email with the post
- it currently returns a 200 with whatever the response of the mail
  service is
- this body isn't used in Admin nor is useful generally because it just
  contains the ID of the mailgun response
- it's better than we change it to 204 and no response
- this commit does that and updates the tests

core/server/api/canary/email-previews.js

@@ -43,7 +43,7 @@ module.exports = {
         }
     },
     sendTestEmail: {
-        statusCode: 200,
+        statusCode: 204,
         headers: {},
         options: [
             'id'

core/server/api/canary/utils/serializers/output/email-previews.js

@@ -1,7 +0,0 @@
-module.exports = {
-    // @TODO remove for 5.0
-    // This should be a default serializer, not a passthrough, as per the read endpoint
-    sendTestEmail(data, apiConfig, frame) {
-        frame.response = data;
-    }
-};

core/server/api/canary/utils/serializers/output/index.js

@@ -109,10 +109,6 @@ module.exports = {
         return require('./site');
     },
 
-    get email_previews() {
-        return require('./email-previews');
-    },
-
     get custom_theme_settings() {
         return require('./custom-theme-settings');
     },

test/e2e-api/admin/email-previews.test.js

@@ -139,10 +139,8 @@ describe('Email Preview API', function () {
                 .send({
                     emails: ['test@ghost.org']
                 })
-                .set('Accept', 'application/json')
-                .expect('Content-Type', /json/)
                 .expect('Cache-Control', testUtils.cacheRules.private)
-                .expect(200)
+                .expect(204)
                 .expect((res) => {
                     res.body.should.be.empty();
                 });
@@ -168,10 +166,8 @@ describe('Email Preview API', function () {
                 .send({
                     emails: ['test@ghost.org']
                 })
-                .set('Accept', 'application/json')
-                .expect('Content-Type', /json/)
                 .expect('Cache-Control', testUtils.cacheRules.private)
-                .expect(200)
+                .expect(204)
                 .expect((res) => {
                     res.body.should.be.empty();
                 });
@@ -199,10 +195,8 @@ describe('Email Preview API', function () {
                 .send({
                     emails: ['test@ghost.org']
                 })
-                .set('Accept', 'application/json')
-                .expect('Content-Type', /json/)
                 .expect('Cache-Control', testUtils.cacheRules.private)
-                .expect(200)
+                .expect(204)
                 .expect((res) => {
                     res.body.should.be.empty();
                 });