Fix regression in test_langs.php

Commit 78cee358 added file Web.config to plugins and lang folder.
test_langs.php treated the file as a plugin folder and a language file.
Merge branch 'fix13-test_langs' of github.com:atrol/mantisbt into atrol-fix13-test_langs
Fix #17510: regression in test_langs.php
Port 68516b0b2fa67112ad1ddc2bfc69dbd25c1bf5df to master:

manage_config_email_page: handle error case for
 array_merge_recursive2

The array_merge_recursive2() function within
manage_config_email_page.php was not performing a check to ensure that
both arrays contain an element with a specified key. This could lead to
warnings and errors being produced when the function is called.