title: Gitlab CI: api trigger with axios call not working when using variables
body: <p>Without the variables the server call works and gitlab is starting the pipeline.</p>
  
  <p>But when I add variables to that call, it errors: "variables needs to be a map of key-valued strings".</p>
  
  <p>This is my code:</p>
  
  <pre><code>    axios
        .post(`https://gitlab.myurl.com/api/v4/projects/${projectId}/trigger/pipeline`, {
          ref: branch,
          token: token,
          variables: { STAGING_AREA: 'testing1', NOTIFY_STATUS: true, SLACK_USER_ID: 'xxxxx' }
        })
        .then(res =&gt; {
          console.log('pipeline started:', res.data.web_url);
        })
        .catch(error =&gt; {
          console.error('errorMessage', error);
        });
  </code></pre>
  
  <p>What is the correct syntax for passing variables?</p>