Using text/plain for error responses

This prevents XSS attacks injecting scripts to error messages. Currently
http error constructor sets content-type to text/html for errors with
the code 500.

Change-Id: I21b5855ebe102060f4046706b5c6ab2e3e532162
Closes-Bug: #1583602
(cherry picked from commit 25fca4bb84)

nailgun/nailgun/api/v1/handlers/base.py

@@ -301,6 +301,7 @@ def content_json(func, cls, *args, **kwargs):
             )
         )
         http_error.data = json_resp(http_error.data)
+        web.header('Content-Type', 'text/plain')
         raise http_error
 
     if all([