date_popup wipes out external validation function

Currently, the date_popup_process() function sets the '#element_validate' part of the form array to be a new array. This wipes out any value that might have been set by the code that generated the form element in the first place. The solution is to treat this as an array, and append the value. If that element of the form array does not already exist, an array is created. See the attached patch.