web_runderjs_ui: serve web script, style and manifest with explicit charset

In HTTP/1.1 , when not specified , charset detault to ISO-8859-1

https://tools.ietf.org/html/rfc2616#section-3.7.1

> media subtypes of the "text"  type are defined to have a default charset value of "ISO-8859-1" when   received via HTTP

Zope adds a charset with default encoding (
https://github.com/zopefoundation/Zope/blob/2.13/src/ZPublisher/HTTPResponse.py#L512-L515 ) but does this only for text/ type, but we serve our javascript as application/javascript

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

@@ -15,13 +15,13 @@ web_content = web_page.getTextContent()
 
 # set headers depending on type of script
 if (portal_type == "Web Script"):
-  response.setHeader('Content-Type', 'application/javascript')
+  response.setHeader('Content-Type', 'application/javascript; charset=utf-8')
 
 elif (portal_type == "Web Style"):
-  response.setHeader('Content-Type', 'text/css')
+  response.setHeader('Content-Type', 'text/css; charset=utf-8')
 
 elif (portal_type == "Web Manifest"):
-  response.setHeader('Content-Type', 'text/cache-manifest')
+  response.setHeader('Content-Type', 'text/cache-manifest; charset=utf-8')
 
 else:
   if (mapping_dict is not None):