Add missing copyright header (go-sql-driver#603)

Author: julienschmidt

driver_go18_test.go

@@ -1,3 +1,11 @@
+// Go MySQL Driver - A MySQL-Driver for Go's database/sql package
+//
+// Copyright 2017 The Go-MySQL-Driver Authors. All rights reserved.
+//
+// This Source Code Form is subject to the terms of the Mozilla Public
+// License, v. 2.0. If a copy of the MPL was not distributed with this file,
+// You can obtain one at http://mozilla.org/MPL/2.0/.
+
 // +build go1.8
 
 package mysql