title: how to add product to sylius cart item from a controller
body: <p>After much fruitless research am still stranded trying to use sylius cart.
  I found out that one way to add item to cart from the twig template is </p>
  
  <pre><code>&lt;form action="{{ path('sylius_cart_item_add', {'id': product.id}) }}" method="post" class="clearfix"&gt;
              {% set form = sylius_cart_form({'product': product}) %}
              {{ form_widget(form.quantity, {'attr': {'class': 'center-text'}, 'empty_value': '1'}) }}
              {{ form_widget(form._token) }}
              &lt;button type="submit" class="btn btn-success"&gt;&lt;i class="icon-shopping-cart icon-white"&gt;&lt;/i&gt; {{ 'sylius.add_to_cart'|trans }}&lt;/button&gt;
          &lt;/form&gt;
  </code></pre>
  
  <p>when I try to use this I get error</p>
  
  <blockquote>
    <p>An exception has been thrown during the rendering of a template ("The option "product" does not exist. Defined options are: "action", "allow_extra_fields", "attr", "auto_initialize", "block_name", "by_reference", "cascade_validation", "compound", "constraints", "csrf_field_name", "csrf_message", "csrf_protection", "csrf_provider", "csrf_token_id", "csrf_token_manager", "data", "data_class", "disabled", "empty_data", "error_bubbling", "error_mapping", "extra_fields_message", "inherit_data", "intention", "invalid_message", "invalid_message_parameters", "label", "label_attr", "label_format", "mapped", "max_length", "method", "pattern", "post_max_size_message", "property_path", "read_only", "required", "translation_domain", "trim", "validation_groups", "virtual".")</p>
  </blockquote>
  
  <p>if I replace the </p>
  
  <pre><code>sylius_cart_form({'product': product})
  </code></pre>
  
  <p>with </p>
  
  <pre><code>sylius_cart_form({'data_class': sylius_cart_item})
  </code></pre>
  
  <p>the error goes out when I submit the quantity does not persist.</p>