Fix http2 agent doesn't work with request module

Fix molnarg#174

Author: felicienfrancois

lib/http.js

@@ -834,7 +834,12 @@ function requestTLS(options, callback) {
   if (options.protocol && options.protocol !== "https:") {
     throw new Error('This interface only supports https-schemed URLs');
   }
-  return (options.agent || exports.globalAgent).request(options, callback);
+  if (options.agent && typeof(options.agent.request) === 'function') {
+    var agentOptions = util._extend({}, options);
+    delete agentOptions.agent;
+    return options.agent.request(agentOptions, callback);
+  }
+  return exports.globalAgent.request(options, callback);
 }
 
 function getRaw(options, callback) {