Merge pull request #7960 from parente/set-no-cache-header

Ensure Cache-Control is set once and only once

IPython/html/base/handlers.py

@@ -404,7 +404,7 b' class FileFindHandler(web.StaticFileHandler):'
         # disable browser caching, rely on 304 replies for savings
         if "v" not in self.request.arguments or \
                 any(self.request.path.startswith(path) for path in self.no_cache_paths):
-            self.add_header("Cache-Control", "no-cache")
+            self.set_header("Cache-Control", "no-cache")
     
     def initialize(self, path, default_filename=None, no_cache_paths=None):
         self.no_cache_paths = no_cache_paths or []