issue: SelectionField nset

This addresses an issue where creating an advanced search with selection
field "does not have a value" throws a fatal error. This was due to an
improperly named search method "notset".

include/class.dynamic_forms.php

@@ -1786,7 +1786,7 @@ class SelectionField extends FormField {
     function getSearchMethods() {
         return array(
             'set' =>        __('has a value'),
-            'notset' =>     __('does not have a value'),
+            'nset' =>     __('does not have a value'),
             'includes' =>   __('includes'),
             '!includes' =>  __('does not include'),
         );
@@ -1795,7 +1795,7 @@ class SelectionField extends FormField {
     function getSearchMethodWidgets() {
         return array(
             'set' => null,
-            'notset' => null,
+            'nset' => null,
             'includes' => array('ChoiceField', array(
                 'choices' => $this->getChoices(),
                 'configuration' => array('multiselect' => true),