Fix URL-decoding to decode + signs

Author: pjcdawkins

src/OAuth2/ClientAssertionType/HttpBasic.php

@@ -120,8 +120,8 @@ public function getClientCredentials(RequestInterface $request, ResponseInterfac
                  * client credentials are URL-encoded before being encoded in the HTTP Basic header, so we decode them here
                  * @see http://tools.ietf.org/html/rfc6749#section-2.3.1
                  */
-                'client_id' => rawurldecode($request->headers('PHP_AUTH_USER')),
-                'client_secret' => rawurldecode($request->headers('PHP_AUTH_PW')),
+                'client_id' => urldecode($request->headers('PHP_AUTH_USER')),
+                'client_secret' => urldecode($request->headers('PHP_AUTH_PW')),
             );
         }