erp5_web_renderjs_ui: WebPage_viewAsWeb now use web page content type

bt5/erp5_web_renderjs_ui/SkinTemplateItem/portal_skins/erp5_web_renderjs_ui/WebPage_viewAsWeb.py

@@ -41,6 +41,6 @@ else:
   # Only fetch code (html, js, css, image) and data from this ERP5, to prevent any data leak as the web site do not control the gadget's code
   response.setHeader("Content-Security-Policy", content_security_policy)
 
-  response.setHeader('Content-Type', 'text/html')
+  response.setHeader('Content-Type', web_page.getContentType('text/html'))
 
 return web_content