title: "ValueError: expected ':' after conversion specifier" when trying to authorise on site with x-api-key
body: <p><strong>Problem:</strong></p>
  
  <ul>
  <li>When I send a yaml file I have crafted from looking at my postman
  request. I get the eror "ValueError: expected ':' after conversion
  specifier" along with a lot of other errors. This seems to be caused
  by the addition of "x-api-key" in the headers as when I take this
  away the error disappears.</li>
  </ul>
  
  <pre><code>---
  test_name: Log in
  
  stages:
   - name: Send log in request
  
      request:
        url: https://myurl.com/auth
        method: POST
        headers:
          Content-Type: application/json
          x-api-key: "&lt;DQDg4lEmau^9{&gt;!Os'mhNP" #Edited this to keep safe but kept the exact same layout
          Authorization: Basic bm90bXlyZWFsZW1haWxAZ21haWwuY29tOnBhc3N3b3JkMTIzCg==
  
      response:
        status_code: 200
  ...
  </code></pre>
  
  <p><strong>Expected Results:</strong></p>
  
  <ul>
  <li>Able to authorise with the server and get JWT Token as I do on
  Postman.</li>
  </ul>
  
  <p><strong>Actual Result:</strong></p>
  
  <ul>
  <li>The script fails with many errors. The main one appears to be the one I listed above however I can provide more if I am wrong about that.</li>
  </ul>
  
  <p><strong>Additional Info:</strong></p>
  
  <ul>
  <li>Removing "x-api-key" lets the request go through however "Forbidden" is returned as this is a required value, I believe.</li>
  </ul>
  
  <p>Thanks for any help, I have been looking through the documentation on Tavern for an hour or two and browsing the internet for the answer. I am new so I am sure it is something simple, but any advice would be great.</p>
  
  <p>Thanks!</p>