issue: Priority Field Template Variable

This addresses an issue with PriorityField Template Variables introduced
with 4359 where the priority variable will appear as "Array" instead of the
actual value. This adds an `asVar()` function to `class PriorityField` to
get the appropriate data.

include/class.forms.php

@@ -2273,6 +2273,10 @@ class PriorityField extends ChoiceField {
         return ($value instanceof Priority) ? array($value->getId()) : null;
     }
 
+    function asVar($value, $id=false) {
+        return $this->to_php($value, $id);
+    }
+
     function getConfigurationOptions() {
         $choices = $this->getChoices();
         $choices[''] = __('System Default');