Updated OAuthToken to throw proper error on not receiving access token

- Updated Samples, Tests

tests/PayPal/Test/Handler/OauthHandlerTest.php

@@ -0,0 +1,73 @@
+<?php
+
+namespace PayPal\Test\Handler;
+
+use PayPal\Core\PayPalHttpConfig;
+use PayPal\Exception\PayPalConfigurationException;
+use PayPal\Rest\ApiContext;
+use PayPal\Auth\OAuthTokenCredential;
+use PayPal\Handler\OauthHandler;
+
+class OauthHandlerTest extends \PHPUnit_Framework_TestCase
+{
+
+    /**
+     * @var \PayPal\Handler\OauthHandler
+     */
+    public $handler;
+
+    /**
+     * @var PayPalHttpConfig
+     */
+    public $httpConfig;
+
+    /**
+     * @var ApiContext
+     */
+    public $apiContext;
+
+    /**
+     * @var array
+     */
+    public $config;
+
+    public function setUp()
+    {
+        $this->apiContext = new ApiContext(
+            new OAuthTokenCredential(
+                'clientId',
+                'clientSecret'
+            )
+        );
+
+    }
+
+    public function modeProvider()
+    {
+        return array(
+            array( array('mode' => 'sandbox') ),
+            array( array('mode' => 'live')),
+            array( array( 'mode' => 'sandbox','oauth.EndPoint' => 'http://localhost/')),
+            array( array('mode' => 'sandbox','service.EndPoint' => 'http://service.localhost/'))
+        );
+    }
+
+
+    /**
+     * @dataProvider modeProvider
+     * @param $configs
+     */
+    public function testGetEndpoint($configs)
+    {
+        $config = $configs + array(
+            'cache.enabled' => true,
+            'http.headers.header1' => 'header1value'
+        );
+        $this->apiContext->setConfig($config);
+        $this->httpConfig = new PayPalHttpConfig(null, 'POST', $config);
+        $this->handler = new OauthHandler($this->apiContext);
+        $this->handler->handle($this->httpConfig, null, $this->config);
+    }
+
+
+}