Update PrettyMin.php

[stemar]

Hello, I like your library, very useful for my project.

I was able to successfully indent all the nodes with spaces with this code patch. Otherwise, it's a mix of "\t" and spaces when I use:

$pm = new Wa72\HtmlPrettymin\PrettyMin(['indent_characters' => "    "]);
$output = $pm->load($html)->indent()->saveHtml();

I hope it's o.k. to suggest this code change. Thank you.