Make github driver code more robust

src/Composer/Repository/Vcs/GitHubDriver.php

@@ -512,8 +512,8 @@ class GitHubDriver extends VcsDriver
     {
         $headers = $this->remoteFilesystem->getLastHeaders();
         foreach ($headers as $header) {
-            if (substr($header, 0, 5) === 'Link:') {
-                $links = explode(',', substr($header, 5));
+            if (preg_match('{^link:\s*(.+?)\s*$}i', $header, $match)) {
+                $links = explode(',', $match[1]);
                 foreach ($links as $link) {
                     if (preg_match('{<(.+?)>; *rel="next"}', $link, $match)) {
                         return $match[1];