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.

Author: rouilj

roundup/cgi/client.py

@@ -2132,7 +2132,8 @@ def renderContext(self):
             except IndexerQueryError as e:
                 result = self.renderError(e.args[0])
 
-            self.additional_headers['Content-Type'] = pt.content_type
+            if 'Content-Type' not in self.additional_headers:
+                self.additional_headers['Content-Type'] = pt.content_type
             if self.env.get('CGI_SHOW_TIMING', ''):
                 if self.env['CGI_SHOW_TIMING'].upper() == 'COMMENT':
                     timings = {'starttag': '<!-- ', 'endtag': ' -->'}