[1.7.x] Fixed an inconsistency introduced in 547b1810.

mark_safe and mark_for_escaping should have been kept similar.

On Python 2 this change has no effect. On Python 3 it fixes the use case
shown in the regression test for mark_for_escaping, which used to raise
a TypeError. The regression test for mark_safe is just for completeness.

Backport of 5c5eb5fe from master.

tests/utils_tests/test_safestring.py

@@ -31,6 +31,15 @@ class SafeStringTest(TestCase):
         self.assertIsInstance(mark_safe(b), SafeData)
         self.assertRenderEqual('{{ s }}', 'a&b', s=mark_safe(s))
 
+    def test_mark_safe_object_implementing_dunder_str(self):
+        class Obj(object):
+            def __str__(self):
+                return '<obj>'
+
+        s = mark_safe(Obj())
+
+        self.assertRenderEqual('{{ s }}', '<obj>', s=s)
+
     def test_mark_for_escaping(self):
         s = mark_for_escaping('a&b')
         self.assertRenderEqual('{{ s }}', 'a&amp;b', s=s)
@@ -47,3 +56,12 @@ class SafeStringTest(TestCase):
     def test_html(self):
         s = '<h1>interop</h1>'
         self.assertEqual(s, mark_safe(s).__html__())
+
+    def test_mark_for_escaping_object_implementing_dunder_str(self):
+        class Obj(object):
+            def __str__(self):
+                return '<obj>'
+
+        s = mark_for_escaping(Obj())
+
+        self.assertRenderEqual('{{ s }}', '&lt;obj&gt;', s=s)