title: How to use a variable in headers when making a POST request in Red
body: <p>I'm trying to make a POST request using Red language.</p>
  
  <p>I have to pass a header with authorization, and I'm computing the value for the authorization string before making the request and saving it in a variable <code>auth-string</code>. </p>
  
  <p><code>probe</code> prints the <code>auth-string</code> value fine.</p>
  
  <p>If I hard-code the authorization string in the headers section, it works, but when I try to use the variable the program exits.</p>
  
  <p>This code with hard-coded <strong><code>Authorization: "Basic c29tZX...</code></strong> works:</p>
  
  <pre><code>username: "someusernamestring"
  password: "somepasswordstring"
  
  url: to url! rejoin ["https://example.com/" username "/Account.json"]
  
  auth-string: rejoin ["Basic " enbase/base rejoin [username ":" password] 64]
  probe auth-string
  
  response: write url [
          post [
          Content-Type: "application/x-www-form-urlencoded"
          Authorization: "Basic c29tZXVzZXJuYW1lc3RyaW5nOnNvbWVwYXNzd29yZHN0cmluZw=="
      ]
      {}    
  ]
  
  print response
  </code></pre>
  
  <hr />
  
  <p>But, this code, with the variable <strong><code>Authorization: auth-string</code></strong> does not work, the program exits, no error that I can see.</p>
  
  <pre><code>username: "someusernamestring"
  password: "somepasswordstring"
  
  url: to url! rejoin ["https://example.com/" username "/Account.json"]
  
  auth-string: rejoin ["Basic " enbase/base rejoin [username ":" password] 64]
  probe auth-string
  
  response: write url [
          post [
          Content-Type: "application/x-www-form-urlencoded"
          Authorization: auth-string
      ]
      {}    
  ]
  
  print response
  </code></pre>
  
  <hr />
  
  <p>How do I use a variable in the headers section? </p>