title: add http request header an WSO2 proxy service
body: <p>I have a proxy service who uses an XSLT mediator, I want to add a request header to it, this information representes a basic HTTP authentication : 
  Authorization:Basic YWRtaW46YWRtaW4=
  The last key is created with a login 'admin' and a password 'admin'.
  How can I add this header to my request in the proxy service as a get request so I can authenticate my proxy before doing the xslt transformation.
  Thanks </p>