WICKET-5602 DynamicImageResource should set the mime type after reading the image data

(cherry picked from commit d0fd2102967f34e35be1d009085c314898b8fb0a)
WICKET-5607 Wicket Ajax fires calls scheduled by AbstractAjaxTimerBehavior even after unload of the page

(cherry picked from commit 4ffd8f6f4986617caf9a4dcf293c9f59daa263a8)