Replace deprecated method to new one

In [Mojolicious 4.77](https://github.com/kraih/mojo/commit/f44ad6868a456241f1cdf2a8ad272cad261da7a1), `to_xml()` method is deprecated and in [4.98](https://github.com/kraih/mojo/commit/fa0f23c11cc9dc92a9bbb3d5846cb0232bcc0c69), it was completely removed from source tree. So humane plugin cannot passed the test with Mojolicious 4.98.

This patch fixes the failed test. :)