FIX does not insert images to activity

When uploading an image with an activity, the html code for the image(s) is not written, the ul list is empty.

RTMediaModel->get() is invoked with a columns array like array('id' => array(123)) - and this results in an invalid sql string WHERE id IN ORDER BY ... => the (123) is missing.

This fix handles this case.
Merge pull request #487 from tobiaskluge/patch-1

FIX does not insert images to activity / invalid sql query id IN ORDER BY