Fixed #13970 -- Made SelectDateWidget use the standard widget is_required attribute

Thanks mitar for the report and Tim Graham for the review.

django/forms/extras/widgets.py

@@ -53,9 +53,8 @@ class SelectDateWidget(Widget):
     day_field = '%s_day'
     year_field = '%s_year'
 
-    def __init__(self, attrs=None, years=None, required=True, months=None):
+    def __init__(self, attrs=None, years=None, months=None):
         self.attrs = attrs or {}
-        self.required = required
 
         # Optional list or tuple of years to use in the "year" select box.
         if years:
@@ -139,7 +138,7 @@ class SelectDateWidget(Widget):
             id_ = self.attrs['id']
         else:
             id_ = 'id_%s' % name
-        if not (self.required and val):
+        if not self.is_required:
             choices.insert(0, self.none_value)
         local_attrs = self.build_attrs(id=field % id_)
         s = Select(choices=choices)