explicitly delete maxAge instead of setting as undefined

Author: jonchurch

Diff for: lib/response.js

@@ -699,7 +699,9 @@ res.get = function(field){
 res.clearCookie = function clearCookie(name, options) {
   // Force cookie expiration by setting expires to the past
   // ensure maxAge is undefined
-  const opts = {path: '/', ...options, expires: new Date(1), maxAge: undefined}
+  const opts = { path: '/', ...options, expires: new Date(1) }
+  // ensure maxAge is not passed
+  delete opts.maxAge
 
   return this.cookie(name, '', opts);
 };