Fixed #6681 -- Don't break docutils when rendering reStructuredText.

Don't set a global default interpreted role function for reStructuredText.
Instead, use the `default-role` directive to change the default only within
the `parse_rst()` function.

Thanks Malcolm Tredinnick for the report.

tests/admin_docs/urls.py

@@ -1,8 +1,9 @@
-from django.conf.urls import patterns
+from django.conf.urls import include, patterns
 
 from . import views
 
 urlpatterns = patterns('',
+    (r'^admindocs/', include('django.contrib.admindocs.urls')),
     (r'^xview/func/$', views.xview_dec(views.xview)),
     (r'^xview/class/$', views.xview_dec(views.XViewClass.as_view())),
 )