Removed unnecessary email_preview.read serializer

refs: https://github.com/TryGhost/Toolbox/issues/245
refs: https://github.com/TryGhost/Ghost/commit/73f91a524

- we don't need this serializer because the default serializer will do the same thing
- commit 73f91a524 fixes the logic so that the default serializer is called as a fallback
  even though the email_preview serialzier exists, as there's no matching method name
- sadly the route name here is wrong, it should be email_previews plural, but the response format is correct
  to make this work we have to fix the docName and rename the serializer

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

@@ -12,7 +12,8 @@ const emailPreview = new mega.EmailPreview({
 });
 
 module.exports = {
-    docName: 'email_preview',
+    // @TODO make the route match the resource name in 5.0
+    docName: 'email_previews',
 
     read: {
         options: [

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

@@ -1,10 +0,0 @@
-module.exports = {
-    read(emailPreview, apiConfig, frame) {
-        frame.response = {
-            email_previews: [emailPreview]
-        };
-    },
-    sendTestEmail(data, apiConfig, frame) {
-        frame.response = data;
-    }
-};

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

@@ -0,0 +1,7 @@
+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

@@ -109,8 +109,8 @@ module.exports = {
         return require('./site');
     },
 
-    get email_preview() {
-        return require('./email-preview');
+    get email_previews() {
+        return require('./email-previews');
     },
 
     get snippets() {

core/server/web/api/canary/admin/routes.js

@@ -290,6 +290,7 @@ module.exports = function apiRoutes() {
     router.get('/actions', mw.authAdminApi, http(api.actions.browse));
 
     // ## Email Preview
+    // @TODO: rename to email_previews in 5.0
     router.get('/email_preview/posts/:id', mw.authAdminApi, http(api.email_preview.read));
     router.post('/email_preview/posts/:id', mw.authAdminApi, http(api.email_preview.sendTestEmail));