[1.7.x] Fixed #23831 -- Supported strings escaped by third-party libs in Django.

Refs #7261 -- Made strings escaped by Django usable in third-party libs.

The changes in mark_safe and mark_for_escaping are straightforward. The
more tricky part is to handle correctly objects that implement __html__.

Historically escape() has escaped SafeData. Even if that doesn't seem a
good behavior, changing it would create security concerns. Therefore
support for __html__() was only added to conditional_escape() where this
concern doesn't exist.

Then using conditional_escape() instead of escape() in the Django
template engine makes it understand data escaped by other libraries.

Template filter |escape accounts for __html__() when it's available.
|force_escape forces the use of Django's HTML escaping implementation.

Here's why the change in render_value_in_context() is safe. Before Django
1.7 conditional_escape() was implemented as follows:

    if isinstance(text, SafeData):
        return text
    else:
        return escape(text)

render_value_in_context() never called escape() on SafeData. Therefore
replacing escape() with conditional_escape() doesn't change the
autoescaping logic as it was originally intended.

This change should be backported to Django 1.7 because it corrects a
feature added in Django 1.7.

Thanks mitsuhiko for the report.

Backport of 6d52f6f from master.

tests/utils_tests/test_safestring.py

@@ -11,6 +11,13 @@ lazystr = lazy(force_text, six.text_type)
 lazybytes = lazy(force_bytes, bytes)
 
 
+class customescape(six.text_type):
+    def __html__(self):
+        # implement specific and obviously wrong escaping
+        # in order to be able to tell for sure when it runs
+        return self.replace('<', '<<').replace('>', '>>')
+
+
 class SafeStringTest(TestCase):
     def assertRenderEqual(self, tpl, expected, **context):
         context = Context(context)
@@ -23,6 +30,14 @@ class SafeStringTest(TestCase):
         self.assertRenderEqual('{{ s }}', 'a&b', s=s)
         self.assertRenderEqual('{{ s|force_escape }}', 'a&amp;b', s=s)
 
+    def test_mark_safe_object_implementing_dunder_html(self):
+        e = customescape('<a&b>')
+        s = mark_safe(e)
+        self.assertIs(s, e)
+
+        self.assertRenderEqual('{{ s }}', '<<a&b>>', s=s)
+        self.assertRenderEqual('{{ s|force_escape }}', '&lt;a&amp;b&gt;', s=s)
+
     def test_mark_safe_lazy(self):
         s = lazystr('a&b')
         b = lazybytes(b'a&b')
@@ -40,11 +55,25 @@ class SafeStringTest(TestCase):
 
         self.assertRenderEqual('{{ s }}', '<obj>', s=s)
 
+    def test_mark_safe_result_implements_dunder_html(self):
+        self.assertEqual(mark_safe('a&b').__html__(), 'a&b')
+
+    def test_mark_safe_lazy_result_implements_dunder_html(self):
+        self.assertEqual(mark_safe(lazystr('a&b')).__html__(), 'a&b')
+
     def test_mark_for_escaping(self):
         s = mark_for_escaping('a&b')
         self.assertRenderEqual('{{ s }}', 'a&amp;b', s=s)
         self.assertRenderEqual('{{ s }}', 'a&amp;b', s=mark_for_escaping(s))
 
+    def test_mark_for_escaping_object_implementing_dunder_html(self):
+        e = customescape('<a&b>')
+        s = mark_for_escaping(e)
+        self.assertIs(s, e)
+
+        self.assertRenderEqual('{{ s }}', '<<a&b>>', s=s)
+        self.assertRenderEqual('{{ s|force_escape }}', '&lt;a&amp;b&gt;', s=s)
+
     def test_mark_for_escaping_lazy(self):
         s = lazystr('a&b')
         b = lazybytes(b'a&b')
@@ -53,10 +82,6 @@ class SafeStringTest(TestCase):
         self.assertIsInstance(mark_for_escaping(b), EscapeData)
         self.assertRenderEqual('{% autoescape off %}{{ s }}{% endautoescape %}', 'a&amp;b', s=mark_for_escaping(s))
 
-    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):