Avoid browser cache  (Return code 304 Not Modified)

If the "plone.app.caching" is configured to cache the images, the image will not be updated in the browser  (return code=304 not modified).

steps to reproduce:
- go to @@caching-controlpanel/caching operations
- set "File and image resources" caching to moderate or strong
- go to @@croppingeditor and crop the image
- After saving the "new" image will not be update in the browser (return code 304).  The user can still force the update with CTRL+F5 Firefox)