title: karate request header validation
body: <p>Would like to be able to assertions to validate that the request header set matches as expected to help validate tests are correctly setting content types supported.</p>
  
  <p>Checked documentation, github issues, stackoverflow, google</p>
  
  <pre>
  Given url myUrl
  And params myParams
  And headers myHeaders
  And header Authorization = babylonAuth
  And request historyPostPayload
  When method POST
  Then status 200
  
  * match requestHeaders.contains({'Content-Type': ''})
  </pre>
  
  <p>Any way to do this now, or is this a feature request?</p>