Add UTCNow() function. (#3931)

This patch adds UTCNow() function which returns current UTC time.

This is equivalent of UTCNow() == time.Now().UTC()

cmd/api-headers_test.go

@@ -1,5 +1,5 @@
 /*
- * Minio Cloud Storage, (C) 2015, 2016 Minio, Inc.
+ * Minio Cloud Storage, (C) 2015, 2016, 2017 Minio, Inc.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -18,12 +18,11 @@ package cmd
 
 import (
 	"testing"
-	"time"
 )
 
 func TestNewRequestID(t *testing.T) {
 	// Ensure that it returns an alphanumeric result of length 16.
-	var id = mustGetRequestID(time.Now().UTC())
+	var id = mustGetRequestID(UTCNow())
 
 	if len(id) != 16 {
 		t.Fail()