Add type property to DefaultSR for stylesheet X-Private-Subreddit check.

r2/r2/models/subreddit.py

@@ -847,6 +847,10 @@ class DefaultSR(_DefaultSR):
     def _fullname(self):
         return "t5_6"
 
+    @property
+    def type(self):
+        return self._base.type if self._base else "public"
+
     @property
     def header(self):
         return (self._base and self._base.header) or _DefaultSR.header