Fix modifying frozen strings error.

app/mailers/user_notifications.rb

@@ -543,7 +543,7 @@ class UserNotifications < ActionMailer::Base
       title = I18n.t("system_messages.private_topic_title", id: post.topic_id)
     end
 
-    context = ""
+    context = +""
     tu = TopicUser.get(post.topic_id, user)
     context_posts = self.class.get_context_posts(post, tu, user)
 

app/models/topic_embed.rb

@@ -36,7 +36,7 @@ class TopicEmbed < ActiveRecord::Base
     if SiteSetting.embed_truncate
       contents = first_paragraph_from(contents)
     end
-    contents ||= ''
+    contents ||= +''
     contents << imported_from_html(url)
 
     url = normalize_url(url)