mu-plugin file that replaces the content of a page with an example demonstrating add_query_arg() does not urlencode parameters.

test-add-query-arg-output.php

<?php
add_action( 'the_content', 'test_add_query_arg_output' );

function test_add_query_arg_output( $content ) {
	return add_query_arg(
		array(
			'var_key' => 'key_value',
			'another_var' => 'My Value'
		),
		home_url()
	);
	// value returned is
	// http://www.example.com?var_key=key_value&another_var=My Value
}