Sync: Respect the fields used in filter_expression of the SyncRule

So that you can filter by fields not used in properties, or in property filters.

fixes #1130
refs #876

library/Director/Import/Sync.php

@@ -283,6 +283,12 @@ class Sync
                     $filterColumns[$column] = $column;
                 }
             }
+            if (($ruleFilter = $this->rule->filter()) !== null) {
+                foreach ($ruleFilter->listFilteredColumns() as $column) {
+                    $filterColumns[$column] = $column;
+                }
+            }
+
             if (! empty($filterColumns)) {
                 foreach (SyncUtils::getRootVariables($filterColumns) as $column) {
                     $usedColumns[$column] = $column;

library/Director/Objects/SyncRule.php

@@ -203,7 +203,10 @@ class SyncRule extends DbObject
         return $this->sync;
     }
 
-    protected function filter()
+    /**
+     * @return Filter
+     */
+    public function filter()
     {
         if ($this->filter === null) {
             $this->filter = Filter::fromQueryString($this->get('filter_expression'));