Use the correct Content-Type header for CAPTCHA

Some security inspection appliances and load balancers don't appreciate
something in the HTTP headers that is not a valid HTTP header. Furthermore,
the browser needs the Content-Type header to identify that the image is not
the PHP default of text/html
Merge pull request #699 from greezybacon/issue/captcha-header

Use the correct Content-Type header for CAPTCHA

Reviewed-By: Peter Rotich <peter@osticket.com>