Update expire date on expireCookie function

According to https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Date and https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Set-Cookie , the date used on expires should be in the same format as Date on HTTP Header.
As of Firefox 60.3.0esr, passing the date in any other format doesn't work and is ignored by the browser.

src/ZPublisher/HTTPResponse.py

@@ -331,7 +331,7 @@ def expireCookie(self, name, **kw):
         if 'value' in d:
             d.pop('value')
         d['max_age'] = 0
-        d['expires'] = 'Wed, 31-Dec-97 23:59:59 GMT'
+        d['expires'] = 'Wed, 31 Dec 1997 23:59:59 GMT'
 
         self.setCookie(name, value='deleted', **d)