http: Support legacy agent.addRequest API

There are some agent subclasses using this today.

Despite the addRequest function being undocumented internal API, it's
easy enough to just support the old signature for backwards
compatibility.

lib/_http_agent.js

@@ -128,6 +128,15 @@ Agent.prototype.getName = function(options) {
 };
 
 Agent.prototype.addRequest = function(req, options) {
+  // Legacy API: addRequest(req, host, port, path)
+  if (typeof options === 'string') {
+    options = {
+      host: options,
+      port: arguments[2],
+      path: arguments[3]
+    };
+  }
+
   var host = options.host;
   var port = options.port;
   var localAddress = options.localAddress;