Merge pull request #214 from cainmi/issue-181

offsetExists() should return true for null values, resolves #181

idiorm.php

@@ -2091,7 +2091,7 @@ public function delete_many() {
         // --------------------- //
 
         public function offsetExists($key) {
-            return isset($this->_data[$key]);
+            return array_key_exists($key, $this->_data);
         }
 
         public function offsetGet($key) {