http: Fix overlooked agent.globalAgent export

Noticed by @bnoordhuis in https://github.com/joyent/node/pull/5991#discussion_r5575946

lib/_http_agent.js

@@ -284,3 +284,5 @@ Agent.prototype.get = function(options, cb) {
   req.end();
   return req;
 };
+
+exports.globalAgent = new Agent();

lib/http.js

@@ -42,8 +42,7 @@ exports.STATUS_CODES = server.STATUS_CODES;
 
 var agent = require('_http_agent');
 var Agent = exports.Agent = agent.Agent;
-var globalAgent = new Agent();
-exports.globalAgent = globalAgent;
+var globalAgent = exports.globalAgent = agent.globalAgent;
 
 var client = require('_http_client');
 var ClientRequest = exports.ClientRequest = client.ClientRequest;