Fix undefined variable error in HttpRequestWrapper

Fixes #19

src/HttpRequestWrapper.php

@@ -110,7 +110,7 @@ public function send(RequestInterface $request, array $options = [])
      */
     public function signRequest(RequestInterface $request)
     {
-        if (!$this->credentials || $credentials['expiry'] < time()) {
+        if (!$this->credentials || $this->credentials['expiry'] < time()) {
             $this->credentials = $this->fetchCredentials();
         }
 

tests/HttpRequestWrapperTest.php

@@ -0,0 +1,46 @@
+<?php
+
+/**
+ * Copyright 2015 Google Inc.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+namespace Google\Gcloud\Tests;
+
+use Google\Gcloud\HttpRequestWrapper;
+use GuzzleHttp\Message\Request as Guzzle5Request;
+use GuzzleHttp\Psr7\Request;
+use Prophecy\Argument;
+
+class HttpRequestWrapperTest extends \PHPUnit_Framework_TestCase
+{
+    public function testSignRequestCanCheckTheExistingCredentialsExpiry()
+    {
+        if (class_exists('GuzzleHttp\\Message\\Request')) {
+            $request = new Guzzle5Request('GET', '/foo');
+        } else {
+            $request = new Request('GET', '/foo');
+        }
+
+        $wrapper = new HttpRequestWrapper();
+
+        // First call loads the application default credentials, and sets an expiry.
+        $wrapper->signRequest($request);
+
+        // The second call _should_ successfully check the existing credentials expiry.
+        $signedRequest = $wrapper->signRequest($request);
+
+        $this->assertStringStartsWith('Bearer', $signedRequest->getHeader('Authorization')[0]);
+    }
+}