fix control flow for second req of pool

Author: Anton Lakin

lib/Pool.js

@@ -25,7 +25,7 @@ Pool.prototype.getConnection = function (cb) {
   if (this._freeConnections.length > 0) {
     connection = this._freeConnections.shift();
 
-    process.nextTick(function(){
+    return process.nextTick(function(){
       return cb(null, connection);
     });
   }