Fix modifying frozen strings error in `OneboxController`.

Fix modifying frozen strings error in OneboxController.

diff --git a/app/controllers/onebox_controller.rb b/app/controllers/onebox_controller.rb
index 02fdabe..04116da 100644
--- a/app/controllers/onebox_controller.rb
+++ b/app/controllers/onebox_controller.rb
@@ -8,7 +8,7 @@ class OneboxController < ApplicationController
   def show
     unless params[:refresh] == 'true'
       preview = Oneboxer.cached_preview(params[:url])
-      preview.strip! if preview.present?
+      preview = preview.strip! if preview.present?
       return render(plain: preview) if preview.present?
     end
 
@@ -31,7 +31,7 @@ class OneboxController < ApplicationController
         topic_id: topic_id
       )
 
-      preview.strip! if preview.present?
+      preview = preview.strip! if preview.present?
 
       Oneboxer.onebox_previewed!(user_id)

GitHub sha: 74989783