[Bug Report] undefined header "Content-Type" for output the captcha challenge

[liaosankai]

Output the captcha challenge on captcha controller is working well on IE browser,but it seem get in trouble on Firefox or Chrome.So I do some fix as follow:

BEFORE:

$this->request->response = Captcha::instance($group)->render(FALSE);

AFTER:

$captcha = Captcha::instance($group)->render(FALSE);
$this->request->headers['Content-Type'] = File::mime($captcha);
$this->request->headers['Content-length'] = filesize($captcha);
$this->request->response = $captcha;

It is working well on all browsers :)

[kolanos]

I've commited this change ff9685e, but it will need to be tested. Ideally the headers should be handled by the class and not the controller based on the captcha type, I magine this breaks text-based captcha.