Return token id if other possible values not present in getTransactionReference.

When processing a request with no card present a token is retrieved from
paypal which is the used by a paypal browser script to open a paypal popup to approve the token.

It's a bit arguable whether this is best retrieved via getTransactionReference or
getCardReference. cardReference makes sense in that the token is used in place of a card
but it's also the only reference we have for the transaction at this point.

Related :
#219

#215

Author: eileenmcnaughton

src/Message/RestResponse.php

@@ -42,6 +42,13 @@ public function getTransactionReference()
             return $this->data['id'];
         }
 
+        if (isset($this->data['token_id'])) {
+            // This would be present when dealing with a case where the user
+            // authorize's their card within paypal. In this case the transactionReference
+            // will be used during the redirect.
+            return $this->data['token_id'];
+        }
+
         return null;
     }
 
@@ -63,6 +70,9 @@ public function getCode()
         return $this->statusCode;
     }
 
+    /**
+     * Get a string that will represent the stored card in future requests.
+     */
     public function getCardReference()
     {
         if (isset($this->data['id'])) {