title: Sharepoint 2013 via REST API: Error 403 Forbidden when trying to create item
body: <p>I'm trying to create a simple list item with the rest api on Sharepoint 2013.
  My code:</p>
  
  <pre><code>$.ajax({
      url: siteUrl + "/_api/web/lists/getByTitle('internal_Listname')/items",
      type: "POST",
      contentType: "application/json;odata=verbose",
      data: JSON.stringify({
           '__metadata': {
              'type': 'SP.Data.internal_ListnameListItem',
           },
           'K1F1': k1f1Result,
      }),
      headers: {
          "accept": "application/json;odata=verbose",
          "X-RequestDigest": $("#__REQUESTDIGEST").val(),
      },
      success: function (data) {
          console.log("done");
      },
      error: function (err) {
          console.log(JSON.stringify(err));
      }
  });
  </code></pre>
  
  <p>When trying to send the data I get the 403 "Forbidden" error.</p>
  
  <pre><code>"error":{
     "code":"-2130575251, Microsoft.SharePoint.SPException",
     "message":{
          "lang":"en-US",
          "value":"The security validation for this page is invalid and might be corrupted. Please use your web browser's Back button to try your operation again."
      }
  }
  </code></pre>
  
  <ul>
  <li>I have full admin privileges on this site and the list.</li>
  </ul>