title: How can I inject a custom HTTP Header into every request that SuperAgent makes?
body: <p>Clearly <a href="https://visionmedia.github.io/superagent/">SuperAgent</a> supports custom HTTP headers:</p>
  
  <pre><code>request
     .post('/api/pet')
     .send({ name: 'Manny', species: 'cat' })
     .set('X-API-Key', 'foobar')
     .set('Accept', 'application/json')
     .end(function(err, res){
       if (res.ok) {
         alert('yay got ' + JSON.stringify(res.body));
       } else {
         alert('Oh no! error ' + res.text);
       }
     });
  </code></pre>
  
  <p>My Question:</p>
  
  <ul>
  <li>If I'm pulling down SuperAgent via npm, how can I inject my own HTTP header across all requests that SuperAgent makes?</li>
  <li>Note: I'm entire willing to create a new npm package that extends SuperAgent if necessary.</li>
  </ul>