Function accepts string or associative array in the params argument.
<?php
$query = new HttpQueryString(false); // query is empty
$query->set(array('city' => 'Prague', 'age' => 34)); // query is "city=Prague&age=34"
$query->set('age=20&gender=male'); // query is "city=Prague&age=20&gender=male"
?>
Also please note that variable can be unset *only* by passing NULL value in an associative array.
<?php
$query->set('city='); // query is "city=&age=20&gender=male"
$query->set(array('city' => null)); // and now "age=20&gender=male" ?>