🐛 Fixed plaintext field not being updated when modifying a post

refs TryGhost/Ghost#9512
- if values for auto-generated fields are passed to Ghost via the API then those values will override any auto-generated value and be saved to the database
- add `plaintext` attribute to list of attributes we strip when serialising to avoid the issue until Ghost has appropriate protections in place

app/serializers/post.js

@@ -34,7 +34,10 @@ export default ApplicationSerializer.extend(EmbeddedRecordsMixin, {
 
         // Properties that exist on the model but we don't want sent in the payload
         delete json.uuid;
+        // Server-side-generated fields
+        // TODO: remove once the API has protection against overriding auto generation
         delete json.html;
+        delete json.plaintext;
         // Inserted locally as a convenience.
         delete json.author_id;
         // Read-only virtual property.