exclude_tree gets overwritten in wp_list_categories

[MikeHansenMe]

When using the function wp_list_categories and passing 'exclude_tree' the value gets overwritten by 'exclude' which by default is . This is because of the following lines in the function:

if ( true == $r['hierarchical'] ) {
	$r['exclude_tree'] = $r['exclude'];
	$r['exclude'] = '';
}

Hierarchical is true by default so unless you set that to false the code gets ran. Unless you pass a value to 'exclude' then the default overwrites 'exclude_tree' which may have been set.

I have included a patch and some unit tests for the wp_list_categories function.

[SergeyBiryukov]

Duplicate of #12981.