Add regression test for problem with mb_encode_mimeheader reported as GH-9683

Author: alexdowad

ext/mbstring/tests/gh9683.phpt

@@ -0,0 +1,11 @@
+--TEST--
+GH-9683 (Problem when ISO-2022-JP-MS is specified in mb_ encode_mimeheader)
+--EXTENSIONS--
+mbstring
+--FILE--
+<?php
+
+echo mb_decode_mimeheader(mb_encode_mimeheader('１２３４５６７８９０１２３４５６７８９０１２', 'ISO-2022-JP-MS', 'B'));
+?>
+--EXPECT--
+１２３４５６７８９０１２３４５６７８９０１２