title: eBay Sell API: Aspects object returns 'Could not serialize field' error, when creating Inventory Item
body: <p>I am using eBay's new REST Sell API to create an inventory item. I am having problems creating product aspects manually. I've tried creating a list of name value pairs but eBay is returning the following error:</p>
  
  <pre><code>Could not serialize field [product.aspects]
  </code></pre>
  
  <p>Below is the request payload sample from eBay:</p>
  
  <pre><code>{
      "availability": {
          "shipToLocationAvailability": {
              "quantity": 50
          }
      },
      "condition": "NEW",
      "product": {
          "title": "GoPro Hero4 Helmet Cam",
          "description": "New GoPro Hero4 Helmet Cam. Unopened box.",
          "aspects": {
              "Brand": [
                  "GoPro"
              ],
              "Type": [
                  "Helmet/Action"
              ],
              "Storage Type": [
                  "Removable"
              ],
              "Recording Definition": [
                  "High Definition"
              ],
              "Media Format": [
                  "Flash Drive (SSD)"
              ],
              "Optical Zoom": [
                  "10x"
              ]
          },
          "imageUrls": [
              "http://i.ebayimg.com/images/i/182196556219-0-1/s-l1000.jpg",
              "http://i.ebayimg.com/images/i/182196556219-0-1/s-l1001.jpg",
              "http://i.ebayimg.com/images/i/182196556219-0-1/s-l1002.jpg"
          ]
      }
  }
  </code></pre>
  
  <p>As far as I know product aspects are not fixed and can be anything, because of this I cannot create a Class. I'm not sure how to handle this apart from manually creating the JSON and inserting it in the correct place in the request payload.</p>
  
  <p>Is there a better way to do this? Maybe create a dynamic object on the fly (any examples would help)?</p>