Fixes the compatibility of XML-RPC library with PHP 8

[gxgpet]

After long thoughts and checks on this, I came up with this fix for the XML-RPC library for the new PHP 8 versions.

While I pushed a unit test and made manual tests by myself, anyone who actively uses this library and can test it with a real-world scenario would be much appreciated.

I don't see the need for the xh property to be a multi-dimensional array. If a new request gets fired, then the previous values won't be used at all.

[tenzap]

Unfortunately the tests report deprecation warnings with php 8.4

Deprecated: xml_set_element_handler(): Passing non-callable strings is deprecated since 8.4 in system/libraries/Xmlrpc.php on line 1167

Deprecated: Function xml_set_object() is deprecated since 8.4, provide a proper method callable to xml_set_*_handler() functions in system/libraries/Xmlrpc.php on line 1165

I also had to change the line in /tests/codeigniter/libraries/Xmlrpc_test.php otherwise phpunit would fail

$this->input = new CI_Input($security);

to

$this->input = new Mock_Core_Input($security, new Mock_Core_Utf8());