title: How to add header to xhr-request
body: <p>I have the following scenario:</p>
  
  <ul>
  <li><p>A startpage that basically includes other pages through xhr-requests.</p></li>
  <li><p>Some of the xhr-requests are behind an firewall with a single-signon-scheme where each http-request to the server is stripped from a set of headers and then populated according to the users grants. </p></li>
  <li>When the request reaches the server it expect some of the reqeusts to have the userId-header. </li>
  <li><p>Test-servers are NOT behind "firewall" so we have a plugin in our browsers to emulate the behaviour.</p>
  
  <p>We would like to run cypress-tests on this page. But we doesn't seem
  to find how to add this userId-header to the xhr-requests and havn't
  been able to find an example of this behaviour. </p>
  
  <p>Could you please provide an example that shows how to do this with a simple example?</p></li>
  </ul>
  
  <p>An attemt to pseduo-code the scenario: 
  cy.visit('myurl').interceptBeforeXhr('xhr-url').addHeader('xhr-url', {'userId' = 'username'}) </p>