hgweb: don't use mutable default argument value

mercurial/hgweb/common.py

@@ -91,12 +91,12 b' permhooks = [checkauthz]'
 
 
 class ErrorResponse(Exception):
-    def __init__(self, code, message=None, headers=[]):
+    def __init__(self, code, message=None, headers=None):
         if message is None:
             message = _statusmessage(code)
         Exception.__init__(self, message)
         self.code = code
-        self.headers = headers
+        self.headers = headers or []
 
 class continuereader(object):
     def __init__(self, f, write):