use a boolean switch to decide if we should serialize

Author: mmorrisson

src/Mailchimp.php

@@ -295,7 +295,7 @@ private static function requestAccessToken($oauth_string)
     {
         $request = self::getStaticRequest();
         $request->setMethod("POST");
-        $request->setPayload($oauth_string);
+        $request->setPayload($oauth_string, false);
         $request->setBaseUrl(MailchimpConnection::TOKEN_REQUEST_URL);
 
         $connection = self::getStaticConnection($request);

src/Requests/MailchimpRequest.php

@@ -265,11 +265,12 @@ public function setAuth()
     /**
      * Sets the payload for a request
      * @param mixed $payload
+     * @param boolean $shouldSerialize
      * @throws MailchimpException when cant serialize payload
      */
-    public function setPayload($payload)
+    public function setPayload($payload, $shouldSerialize = true))
     {
-        if (is_array($payload)) {
+        if ($shouldSerialize) {
             $payload = $this->serializePayload($payload);
         }
         $this->payload = $payload;