fix #139 due date based workflows

I think this sorts it out without bad side effects

modules/Calendar/models/Field.php

@@ -45,6 +45,9 @@ class Calendar_Field_Model extends Vtiger_Field_Model {
* @return <String> Data type of the field
*/
public function getFieldDataType() {
+                if ($this->getModule()->getName()=="Calendar" && $this->getName() == 'due_date'){
+                        return 'date';
+                }
if($this->getName() == 'date_start' || $this->getName() == 'due_date') {
return 'datetime';
} else if($this->get('uitype') == '30') {