adding target subject to the generate access token call

lib/PayPal/Auth/OAuthTokenCredential.php

@@ -47,6 +47,11 @@ class OAuthTokenCredential extends PayPalResourceModel
      */
     private $clientSecret;
 
+    /**
+     * Target subject
+     */
+    private $targetSubject;
+
     /**
      * Generated Access Token
      *
@@ -81,11 +86,12 @@ class OAuthTokenCredential extends PayPalResourceModel
      * @param string $clientId     client id obtained from the developer portal
      * @param string $clientSecret client secret obtained from the developer portal
      */
-    public function __construct($clientId, $clientSecret)
+    public function __construct($clientId, $clientSecret, $targetSubject = null)
     {
         $this->clientId = $clientId;
         $this->clientSecret = $clientSecret;
         $this->cipher = new Cipher($this->clientSecret);
+        $this->targetSubject = $targetSubject;
     }
 
     /**
@@ -267,6 +273,9 @@ private function generateAccessToken($config, $refreshToken = null)
             $params['grant_type'] = 'refresh_token';
             $params['refresh_token'] = $refreshToken;
         }
+        if ($this->targetSubject != null) {
+            $params['target_subject'] = $this->targetSubject;
+        }
         $payload = http_build_query($params);
         $response = $this->getToken($config, $this->clientId, $this->clientSecret, $payload);