[1.11.x] Fixed #27866 -- Made ChoiceWidget.format_value() return a list

Thanks Tim Graham for the review.
Backport of e487ffd3f0 from master.

django/forms/widgets.py

@@ -647,16 +647,10 @@ class ChoiceWidget(Widget):
         return getter(name)
 
     def format_value(self, value):
-        """Return selected values as a set."""
+        """Return selected values as a list."""
         if not isinstance(value, (tuple, list)):
             value = [value]
-        values = set()
-        for v in value:
-            if v is None:
-                values.add('')
-            else:
-                values.add(force_text(v))
-        return values
+        return [str(v) if v is not None else '' for v in value]
 
 
 class Select(ChoiceWidget):

tests/forms_tests/widget_tests/test_selectmultiple.py

@@ -7,6 +7,12 @@ class SelectMultipleTest(WidgetTest):
     widget = SelectMultiple
     numeric_choices = (('0', '0'), ('1', '1'), ('2', '2'), ('3', '3'), ('0', 'extra'))
 
+    def test_format_value(self):
+        widget = self.widget(choices=self.numeric_choices)
+        self.assertEqual(widget.format_value(None), [''])
+        self.assertEqual(widget.format_value(''), [''])
+        self.assertEqual(widget.format_value([3, 0, 1]), ['3', '0', '1'])
+
     def test_render_selected(self):
         self.check_html(self.widget(choices=self.beatles), 'beatles', ['J'], html=(
             """<select multiple="multiple" name="beatles">