[1.6.x] Ensured that BoundField.as_widget always returns properly localized fields.

This is a follow-up to #18777 which improperly converted to strings in
prepare_value and as such caused regressions like #21074.

Refs #18777, #21074

Backport of 56743cf9e3 from master.

django/forms/fields.py

@@ -115,8 +115,6 @@ class Field(object):
         super(Field, self).__init__()
 
     def prepare_value(self, value):
-        if self.widget.is_localized:
-            value = formats.localize_input(value)
         return value
 
     def to_python(self, value):
@@ -459,7 +457,6 @@ class DateTimeField(BaseTemporalField):
     }
 
     def prepare_value(self, value):
-        value = super(DateTimeField, self).prepare_value(value)
         if isinstance(value, datetime.datetime):
             value = to_current_timezone(value)
         return value

django/forms/forms.py

@@ -457,6 +457,9 @@ class BoundField(object):
         if not widget:
             widget = self.field.widget
 
+        if self.field.localize:
+            widget.is_localized = True
+
         attrs = attrs or {}
         auto_id = self.auto_id
         if auto_id and 'id' not in attrs and 'id' not in widget.attrs: