Merge branch 'hotfix/94'

Close #94
Fixes #92
Fixes zfcampus/zf-apigility-skeleton#104

src/Authentication/HttpAdapter.php

@@ -120,6 +120,11 @@ public function preAuth(Request $request, Response $response)
      */
     public function authenticate(Request $request, Response $response, MvcAuthEvent $mvcAuthEvent)
     {
+        if (! $request->getHeader('Authorization', false)) {
+            // No credentials were present at all, so we just return a guest identity.
+            return new Identity\GuestIdentity();
+        }
+
         $this->httpAuth->setRequest($request);
         $this->httpAuth->setResponse($response);