Fixed #31404 -- Changed selector-chosen's multiple attribute to HTML5 boolean syntax.

AUTHORS

@@ -954,6 +954,7 @@ answer newbie questions, and generally made Django that much better:
     zegor
     Zeynel Özdemir <ozdemir.zynl@gmail.com>
     Zlatko Mašek <zlatko.masek@gmail.com>
+    zriv <https://github.com/zriv>
     <Please alphabetize new entries>
 
 A big THANK YOU goes to:

django/contrib/admin/static/admin/js/SelectFilter2.js

@@ -95,7 +95,7 @@ Requires core.js and SelectBox.js.
                 )
             );
 
-            var to_box = quickElement('select', selector_chosen, '', 'id', field_id + '_to', 'multiple', 'multiple', 'size', from_box.size, 'name', from_box.name);
+            var to_box = quickElement('select', selector_chosen, '', 'id', field_id + '_to', 'multiple', '', 'size', from_box.size, 'name', from_box.name);
             to_box.className = 'filtered';
             var clear_all = quickElement('a', selector_chosen, gettext('Remove all'), 'title', interpolate(gettext('Click to remove all chosen %s at once.'), [field_name]), 'href', '#', 'id', field_id + '_remove_all_link');
             clear_all.className = 'selector-clearall';

js_tests/admin/SelectFilter2.test.js

@@ -15,6 +15,7 @@ QUnit.test('init', function(assert) {
         $('.selector-available select').outerHeight() + $('.selector-filter').outerHeight(),
         $('.selector-chosen select').height()
     );
+    assert.equal($('.selector-chosen select')[0].getAttribute('multiple'), '');
     assert.equal($('.selector-chooseall').text(), "Choose all");
     assert.equal($('.selector-add').text(), "Choose");
     assert.equal($('.selector-remove').text(), "Remove");