moodle_url has no way to set the url scheme

The scheme can be set in the constructor but not changed afterwards. Seems like simply an omission in the moodle_url api as it is an edge case, but it is still quite valid and means you don't have to construct a url, encode it, and then re-parse it and replace the scheme afterwards.

require_once('config.php');

$url = new moodle_url('/local/blah/someapi');
$url->set_scheme('myscheme');

echo $url; --> "myscheme://moodle.example.com/local/blah/someapi"