set content-type=..;charset header based on the output_encoding

lib/python/Products/PageTemplates/www/ptEdit.zpt

-<h1 tal:replace="structure python:context.manage_page_header(management_page_charset='utf-8')">Header</h1>
+<h1 tal:replace="structure python:context.manage_page_header(management_page_charset=context.output_encoding)">Header</h1>
 <h2 tal:define="manage_tabs_message options/manage_tabs_message | nothing"
     tal:replace="structure context/manage_tabs">Tabs</h2>