Fix #11011: Can't add custom fields with UTF8 names

Custom fields with UTF8 names could not be created or managed without
errors being experienced. This was due to use of ASCII string functions
instead of UTF8 functions.

Signed-off-by: David Hicks <hickseydr@optusnet.com.au>

core/columns_api.php

@@ -358,7 +358,7 @@ function column_get_title( $p_column ) {
  * @access public
  */
 function columns_ensure_valid( $p_field_name, $p_columns_to_validate, $p_columns_all ) {
-	$t_columns_all_lower = array_map( 'strtolower', $p_columns_all );
+	$t_columns_all_lower = array_map( 'utf8_strtolower', $p_columns_all );
 
 	# Check for invalid fields
 	foreach( $p_columns_to_validate as $t_column ) {
@@ -394,7 +394,7 @@ function columns_ensure_valid( $p_field_name, $p_columns_to_validate, $p_columns
  * @access public
  */
 function columns_remove_invalid( $p_columns, $p_columns_all ) {
-	$t_columns_all_lower = array_values( array_map( 'strtolower', $p_columns_all ) );
+	$t_columns_all_lower = array_values( array_map( 'utf8_strtolower', $p_columns_all ) );
 	$t_columns = array();
 
 	foreach( $p_columns as $t_column ) {