Fix #67 Fixed Cookie creation in cookie plugin.

Fix applied by calculating the expireIn (max-age) seconds instead of expire datetime.
Expanded save spec by using a more complete cookie.

Use DATE_COOKIE constant instead of DateTime constant.
Expand the cookie header content with all parts.

Author: Peter Bouwdewijn

spec/CookiePluginSpec.php

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

src/CookiePlugin.php

@@ -109,11 +109,12 @@ private function createCookie(RequestInterface $request, $setCookie)
 
             switch (strtolower($key)) {
                 case 'expires':
-                    $expires = \DateTime::createFromFormat(DATE_COOKIE, $value);
+                    $expires = \DateTime::createFromFormat(DATE_COOKIE, $value)->getTimestamp()
+                        - (new \DateTime())->getTimestamp();
                     break;
 
                 case 'max-age':
-                    $expires = (new \DateTime())->add(new \DateInterval('PT'.(int) $value.'S'));
+                    $expires = (int) $value;
                     break;
 
                 case 'domain':