Description:
------------
Some per-dir or runtime settings that is set in a request pollutes the global state of the extension and they leak into another request when the web server's forked instances are reused between connections and no corresponding system settings are provded.


Reproduce code:
---------------
<?php
// run without any system settings.

// if the language is set to neutral,
// the default value for the internal encoding is ISO-8859-1
var_dump(mb_internal_encoding());
ini_set('mbstring.internal_encoding', 'UTF-8');
var_dump(mb_internal_encoding());
?>


Expected result:
----------------
The first request:
ISO-8859-1
UTF-8

The second request:
ISO-8859-1
UTF-8

Actual result:
--------------
The first request:
ISO-8859-1
UTF-8

The second request:
UTF-8
UTF-8