[1.8.x] Fixed #24252 -- Forced lazy __str__ to utf-8 on Python 2

Thanks Stanislas Guerra for the report and Tomas Ehrlich for
the review.
Backport of cd0ceaa102 from master.

django/utils/functional.py

@@ -120,6 +120,7 @@ def lazy(func, *resultclasses):
                     cls.__str__ = cls.__text_cast
                 else:
                     cls.__unicode__ = cls.__text_cast
+                    cls.__str__ = cls.__bytes_cast_encoded
             elif cls._delegate_bytes:
                 if six.PY3:
                     cls.__bytes__ = cls.__bytes_cast
@@ -142,6 +143,9 @@ def lazy(func, *resultclasses):
         def __bytes_cast(self):
             return bytes(func(*self.__args, **self.__kw))
 
+        def __bytes_cast_encoded(self):
+            return func(*self.__args, **self.__kw).encode('utf-8')
+
         def __cast(self):
             if self._delegate_bytes:
                 return self.__bytes_cast()

tests/urlpatterns_reverse/tests.py

@@ -318,6 +318,17 @@ class ReverseLazyTest(TestCase):
         response = self.client.get('/login_required_view/')
         self.assertEqual(response.status_code, 200)
 
+    def test_inserting_reverse_lazy_into_string(self):
+        self.assertEqual(
+            'Some URL: %s' % reverse_lazy('some-login-page'),
+            'Some URL: /login/'
+        )
+        if six.PY2:
+            self.assertEqual(
+                b'Some URL: %s' % reverse_lazy('some-login-page'),
+                'Some URL: /login/'
+            )
+
 
 class ReverseLazySettingsTest(AdminScriptTestCase):
     """