[1.11.x] Fixed #28265 -- Prevented renderer warning on Widget.render() with **kwargs.

Backport of 29a518006f from master

tests/forms_tests/widget_tests/test_render_deprecation.py

@@ -0,0 +1,30 @@
+from django import forms
+from django.test import SimpleTestCase
+from django.utils.deprecation import RemovedInDjango21Warning
+
+
+class RenderDeprecationTests(SimpleTestCase):
+    def test_custom_widget_renderer_warning(self):
+        class CustomWidget1(forms.TextInput):
+            def render(self, name, value, attrs=None, renderer=None):
+                return super(CustomWidget1, self).render(name, value, attrs, renderer)
+
+        class CustomWidget2(forms.TextInput):
+            def render(self, *args, **kwargs):
+                return super(CustomWidget2, self).render(*args, **kwargs)
+
+        class CustomWidget3(forms.TextInput):
+            def render(self, name, value, attrs=None):
+                return super(CustomWidget3, self).render(name, value, attrs)
+
+        class MyForm(forms.Form):
+            foo = forms.CharField(widget=CustomWidget1)
+            bar = forms.CharField(widget=CustomWidget2)
+            baz = forms.CharField(widget=CustomWidget3)
+
+        form = MyForm()
+        str(form['foo'])  # No warning.
+        str(form['bar'])  # No warning.
+        msg = "Add the `renderer` argument to the render() method of <class "
+        with self.assertRaisesMessage(RemovedInDjango21Warning, msg):
+            str(form['baz'])