fix another [DEP0066] warning triggered in agent. Still some work needed on tests to support node 12.x

Author: Florentin Thullier

lib/spdy/agent.js

@@ -248,7 +248,7 @@ proto._createStream = function _createStream (req, handle) {
   return state.connection.reserveStream({
     method: req.method,
     path: req.path,
-    headers: req._headers,
+    headers: req.getHeaders ? req.getHeaders() : req._headers,
     host: state.host
   }, function (err, stream) {
     if (err) {