Merge pull request #4858 from Matt-Yorkley/3-0-embedded

[Spree 2.1] Fix embedded response headers

app/services/embedded_page_service.rb

@@ -49,7 +49,8 @@ class EmbeddedPageService
   end
 
   def set_response_headers
-    @response.headers.delete 'X-Frame-Options'
+    @response.headers.except! 'X-Frame-Options'
+    @response.default_headers.except! 'X-Frame-Options'
     @response.headers['Content-Security-Policy'] = "frame-ancestors 'self' #{@embedding_domain}"
   end