Clear context error on post creation when create_at is set (#4615)

api/post.go

@@ -134,6 +134,7 @@ func CreatePost(c *Context, post *model.Post, triggerWebhooks bool) (*model.Post
 
 	if post.CreateAt != 0 && !HasPermissionToContext(c, model.PERMISSION_MANAGE_SYSTEM) {
 		post.CreateAt = 0
+		c.Err = nil
 	}
 
 	post.Hashtags, _ = model.ParseHashtags(post.Message)

webapp/client/client.jsx

@@ -1508,7 +1508,7 @@ export default class Client {
             set(this.defaultHeaders).
             type('application/json').
             accept('application/json').
-            send(post).
+            send({...post, create_at: 0}).
             end(this.handleResponse.bind(this, 'createPost', success, error));
 
         this.track('api', 'api_posts_create', post.channel_id, 'length', post.message.length);