title: Prestashop add product error : Property Product->link_rewrite is empty
body: <p>hello i'm tring to add new product to my prestashop store using REST api , but when i send my xml request i have this response: Validation error: "Property Product->link_rewrite is empty" !!! 
  this is my xml request :</p>
  
  <pre><code>xml=&lt;?xml version="1.0" encoding="UTF-8"?&gt;
  &lt;prestashop xmlns:xlink="http://www.w3.org/1999/xlink"&gt;
  &lt;product&gt;
      &lt;id/&gt;
      &lt;id_manufacturer/&gt;
      &lt;id_supplier/&gt;
      &lt;id_category_default/&gt;
      &lt;new/&gt;
      &lt;cache_default_attribute/&gt;
      &lt;id_default_image/&gt;
      &lt;id_default_combination/&gt;
      &lt;id_tax_rules_group/&gt;
      &lt;type/&gt;
      &lt;id_shop_default/&gt;
      &lt;reference/&gt;
      &lt;supplier_reference/&gt;
      &lt;location/&gt;
      &lt;width/&gt;
      &lt;height/&gt;
      &lt;depth/&gt;
      &lt;weight/&gt;
      &lt;quantity_discount/&gt;
      &lt;ean13/&gt;
      &lt;upc/&gt;
      &lt;cache_is_pack/&gt;
      &lt;cache_has_attachments/&gt;
      &lt;is_virtual/&gt;
      &lt;on_sale/&gt;
      &lt;online_only/&gt;
      &lt;ecotax/&gt;
      &lt;minimal_quantity/&gt;
      &lt;price&gt;1000&lt;/price&gt;
      &lt;wholesale_price/&gt;
      &lt;unity/&gt;
      &lt;unit_price_ratio/&gt;
      &lt;additional_shipping_cost/&gt;
      &lt;customizable/&gt;
      &lt;text_fields/&gt;
      &lt;uploadable_files/&gt;
      &lt;active&gt;1&lt;/active&gt;
      &lt;redirect_type/&gt;
      &lt;id_product_redirected/&gt;
      &lt;available_for_order/&gt;
      &lt;available_date/&gt;
      &lt;condition/&gt;
      &lt;show_price/&gt;
      &lt;indexed/&gt;
      &lt;visibility/&gt;
      &lt;advanced_stock_management/&gt;
      &lt;date_add/&gt;
      &lt;date_upd/&gt;
      &lt;meta_description&gt;&lt;language id="1"/&gt;&lt;language id="2"/&gt;&lt;/meta_description&gt;
      &lt;meta_keywords&gt;&lt;language id="1"/&gt;&lt;language id="2"/&gt;&lt;/meta_keywords&gt;
      &lt;meta_title&gt;&lt;language id="1"/&gt;&lt;language id="2"/&gt;&lt;/meta_title&gt;
      &lt;link_rewrite&gt;hello&lt;/link_rewrite&gt;
      &lt;name&gt;test name&lt;/name&gt;
      &lt;description&gt;test description&lt;/description&gt;
  
      &lt;available_now&gt;&lt;language id="1"/&gt;&lt;language id="2"/&gt;&lt;/available_now&gt;
      &lt;available_later&gt;&lt;language id="1"/&gt;&lt;language id="2"/&gt;&lt;/available_later&gt;
  &lt;associations&gt;&lt;/associations&gt;
  &lt;/product&gt;
  &lt;/prestashop&gt;
  </code></pre>
  
  <p>please i'm waiting for any help </p>