Fixed #7261 -- support for __html__ for library interoperability

The idea is that if an object implements __html__ which returns a string this is
used as HTML representation (eg: on escaping). If the object is a str or unicode
subclass and returns itself the object is a safe string type.

This is an updated patch based on jbalogh and ivank patches.

django/utils/html.py

@@ -65,8 +65,8 @@ def conditional_escape(text):
     """
     Similar to escape(), except that it doesn't operate on pre-escaped strings.
     """
-    if isinstance(text, SafeData):
-        return text
+    if hasattr(text, '__html__'):
+        return text.__html__()
     else:
         return escape(text)