incorrect way to check if user is logged in as guest

[danmarsden]

https://github.com/learnweb/moodle-local_chunkupload/blob/master/classes/chunkupload_form_element.php#L240

better to use the core api "isguestuser()"

and... that also makes me think that your require_login_in_context_ajax should be forcing login even when $cfg->forcelogin is not set. (Issue #10)

[justusdieckmann]

Fixed by #15, thanks :)