http: add callback is function check

We were checking that the callback existed, but not
checking that it was a function. In `setTimeout`, if
callback is truthy but not a function, throw a
TypeError

Reviewed-By: Sakthipriyan Vairamani <thechargingvolcano@gmail.com>
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
PR-URL: https://github.com/nodejs/node/pull/3090

lib/_http_outgoing.js

@@ -89,8 +89,12 @@ exports.OutgoingMessage = OutgoingMessage;
 
 
 OutgoingMessage.prototype.setTimeout = function(msecs, callback) {
-  if (callback)
+
+  if (callback) {
+    if (typeof callback !== 'function')
+      throw new TypeError('callback must be a function');
     this.on('timeout', callback);
+  }
 
   if (!this.socket) {
     this.once('socket', function(socket) {