TimerServer: use an int as SERVER_PORT in environment

Since https://github.com/zopefoundation/Zope/commit/2da2c2c28a8b21e78e8ef93aa17da926e3102c26
SERVER_PORT is assumed to be convertible to int.

The value of this port is not really supposed to be used because
TimerServer requests does not have a server (for example in CMFActivity
we recreate the initial request), so put a value of -1 to prevent
accidental use.

product/TimerService/timerserver/TimerServer.py

@@ -110,7 +110,7 @@ TIMER_ENVIRON = {
     'REQUEST_METHOD': 'GET',
     'SERVER_SOFTWARE': 'TimerServer for Zope',
     'SERVER_NAME': '',
-    'SERVER_PORT': '',
+    'SERVER_PORT': '-1',
     'REMOTE_ADDR': '',
     'GATEWAY_INTERFACE': 'CGI/1.1',
     'SERVER_PROTOCOL': 'HTTP/1.0',