http: use an unref'd timer to fix delay in exit

There was previously up to a second exit delay when exiting node
right after an http request/response, due to the utcDate() function
doing a setTimeout to update the cached date/time.

Fixing this should increase the performance of our http tests.

lib/_http_outgoing.js

@@ -21,6 +21,7 @@
 
 var assert = require('assert').ok;
 var Stream = require('stream');
+var timers = require('timers');
 var util = require('util');
 
 var common = require('_http_common');
@@ -44,12 +45,14 @@ function utcDate() {
   if (!dateCache) {
     var d = new Date();
     dateCache = d.toUTCString();
-    setTimeout(function() {
-      dateCache = undefined;
-    }, 1000 - d.getMilliseconds());
+    timers.enroll(utcDate, 1000 - d.getMilliseconds());
+    timers._unrefActive(utcDate);
   }
   return dateCache;
 }
+utcDate._onTimeout = function() {
+  dateCache = undefined;
+};
 
 
 function OutgoingMessage() {