wsgi: Make it easier to customise response headers in subclasses.

caucase/wsgi.py

@@ -36,9 +36,11 @@ class ApplicationError(Exception):
   WSGI HTTP error base class.
   """
   status = _getStatus(httplib.INTERNAL_SERVER_ERROR)
+  _response_headers = []
+
   @property
   def response_headers(self):
-    return []
+    return self._response_headers[:]
 
 class BadRequest(ApplicationError):
   """