Hard-code sent timezone in If-Modified-Since header

If-Modified-Since format must be RFC850 compliant and in GMT timezone
BUT PHP DATE_RFC850 format keep returning UTC timezone. So, we set the
GMT timezone and force its rendering in the header.

Author: Gildas Quéméner

lib/Github/HttpClient/CachedHttpClient.php

@@ -67,9 +67,9 @@ protected function createRequest($httpMethod, $url)
 
         if ($modifiedAt = $this->getCache()->getModifiedSince($url)) {
             $modifiedAt = new \DateTime('@'.$modifiedAt);
-            $modifiedAt->setTimezone(new \DateTimeZone('GMT')); 
+            $modifiedAt->setTimezone(new \DateTimeZone('GMT'));
 
-            $request->addHeader(sprintf('If-Modified-Since: %s', $modifiedAt->format(DATE_RFC2822)));
+            $request->addHeader(sprintf('If-Modified-Since: %s GMT', $modifiedAt->format('l, d-M-y H:i:s')));
         }
 
         return $request;