fix: if Content-Type header defined, don't overwrite with default When processing a template, you should be able to set the content-type header using: tal:replace="python:request.client.additional_headers.update( {'Content-Type':'application/vnd.roundup.translation+json'} )" The client used to always overwrite it with the content type derived from the template extension or text/html for unknown extensions. This patch stops the overwriting if the Content-Type is already set in the header. This particular use case was to use an XML document to process a url like: /tracker?@template=translation+json&tokens=Login,Welcome+%s and get back json data of the translated strings. If the file was named '_generic.json' and we used '@template=json' we got application/json returned. For other template names we got text/html.