Invalid regexps in header_filters cause shunted messages
For each header_filters rule, mailman.chains.headers.check()
collects all the matching headers and then does re.search()
for the pattern. If the pattern is not a valid regexp, this throws an uncaught exception and the message is shunted.
The search should by in a try
and exceptions caught and logged. Also, REST
should validate the regexp before storing it which it currently doesn't do.