Fix #67 Use fixed expires date string. +100 years should suffice.

Author: Peter Bouwdewijn

spec/CookiePluginSpec.php

@@ -146,12 +146,7 @@ function it_saves_cookie(RequestInterface $request, ResponseInterface $response,
 
         $response->hasHeader('Set-Cookie')->willReturn(true);
         $response->getHeader('Set-Cookie')->willReturn([
-            sprintf(
-                'cookie=value; expires=%s; Max-Age=60; path=/; domain=test.com; secure; HttpOnly',
-                (new \DateTime())->add(
-                    new \DateInterval('PT60S')
-                )->format(\DateTime::COOKIE)
-            )
+            'cookie=value; expires=Wednesday, 31-Mar-2116 06:34:54 UTC; Max-Age=60; path=/; domain=test.com; secure; HttpOnly',
         ]);
 
         $request->getUri()->willReturn($uri);