title: Vend Api create or update product returns 500 error
body: <p>I need to add product to my vend account via Api, but it seams not working.
  I am using Yii2 for developing, I have send request with Curl and Yii request objects.</p>
  
  <p><strong>Curl request</strong></p>
  
  <pre><code>$ch = curl_init() ;
  curl_setopt( $ch, CURLOPT_URL, $vend_api ) ;
  curl_setopt( $ch, CURLOPT_CUSTOMREQUEST, "POST" ) ;
  curl_setopt( $ch, CURLOPT_POSTFIELDS, $product ) ;
  curl_setopt( $ch, CURLOPT_HTTPHEADER, array( 'Authorization: Bearer '.$user['token'] ) ) ;
  curl_setopt( $ch, CURLOPT_RETURNTRANSFER, 1 ) ;
  curl_setopt( $ch, CURLOPT_SSL_VERIFYPEER, false ) ;
  $result = curl_exec( $ch ) ;
  </code></pre>
  
  <p><strong>Yii2 Request</strong></p>
  
  <pre><code>$client = new Client();
  $response = $client-&gt;createRequest()
      -&gt;setMethod('post')
      -&gt;setUrl($vend_api)
      -&gt;addHeaders(['Authorization' =&gt; 'Bearer ' . $user['token']])
      -&gt;setData($product);
     -&gt;send();
  </code></pre>
  
  <p>Both returns 500 internal server error of Vend</p>
  
  <p><a href="https://i.stack.imgur.com/F80Fo.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/F80Fo.png" alt="enter image description here"></a></p>