fix(filtering): transform letters to locale specific cases (#1511)

docs/api/props.md

@@ -189,7 +189,7 @@ disabled: {
 
 ## dropdownShouldOpen <Badge text="v3.12.0+" />
 
-Determines whether the dropdown should open. Used 
+Determines whether the dropdown should open. Used
 for overriding the default dropdown behaviour. Receives
 the vue-select instance as the single argument to the function.
 
@@ -250,7 +250,7 @@ if the option should be displayed.
 filterBy: {
   type: Function,
   default(option, label, search) {
-    return (label || '').toLowerCase().indexOf(search.toLowerCase()) > -1
+    return (label || '').toLocaleLowerCase().indexOf(search.toLocaleLowerCase()) > -1
   }
 },
 ```