Sockets should never be attached to a ClientRequest before nextTick().

This way the API for interacting directly with the socket object is
consistent before and after the Agent pool is exhausted.

Fixes #1601.

lib/http2.js

@@ -1070,9 +1070,9 @@ ClientRequest.prototype.abort = function() {
 };
 
 ClientRequest.prototype.onSocket = function(socket) {
-  var parser = parsers.alloc();
   var req = this;
-
+  process.nextTick(function () {
+    var parser = parsers.alloc();
     req.socket = socket;
     req.connection = socket;
     parser.reinitialize('response');
@@ -1209,9 +1209,9 @@ ClientRequest.prototype.onSocket = function(socket) {
 
       return isHeadResponse;
     };
-  process.nextTick(function() {
     req.emit('socket', socket);
   });
+
 };
 ClientRequest.prototype._deferToConnect = function(method, arguments, cb) {
   // This function is for calls that need to happen once the socket is