Fix deprecated dynamic property in Batch class (#32)

Author: OndraFiedler

README.md

@@ -17,7 +17,7 @@ or
 ```
 {
     "require": {
-        "recombee/php-api-client": "^4.1.1"
+        "recombee/php-api-client": "^4.1.2"
     }
 }
 ```

src/RecommApi/Client.php

@@ -86,7 +86,7 @@ protected function getBaseUri() {
     }
 
     protected function getUserAgent() {
-        $user_agent = 'recombee-php-api-client/4.1.1';
+        $user_agent = 'recombee-php-api-client/4.1.2';
         if (isset($this->options['serviceName']))
             $user_agent .= ' '.($this->options['serviceName']);
         return $user_agent;

src/RecommApi/Requests/Batch.php

@@ -24,6 +24,11 @@ class Batch extends Request {
      */ 
     public $requests;
 
+    /**
+     * @var array Array containing values of optional parameters
+     */
+    protected $optional;
+
     /**
      * Construct the Batch request
      * @param Request[] $requests Array of Requests.