title: How modify the request headers using safari extension
body: <p>I am developing a safari extension which will modify the http request headers.
  I am able to modify the request headers in chrome and firefox extensions but not in safari extension.
  In chrome there is <a href="https://developer.chrome.com/extensions/webRequest" rel="nofollow">webRequest</a> API to modify the request header. 
  chrome.webRequest.onHeadersReceived.addListener(function (details));</p>
  
  <p>In firefox by using the "http-on-examine-response" http Request Observer
  Any help or clues are mostly appreciated</p>