Test for RegexURLPattern.callback on Django 1.10

Thanks Paul J Stevens for the initial patch, Tim Graham for review
and Matt Westcott for tweak of initial patch

wagtail/utils/urlpatterns.py

@@ -1,14 +1,35 @@
 from __future__ import absolute_import, unicode_literals
-
 from functools import update_wrapper
+from django import VERSION as DJANGO_VERSION
 
 
 def decorate_urlpatterns(urlpatterns, decorator):
+    """Decorate all the views in the passed urlpatterns list with the given decorator"""
     for pattern in urlpatterns:
         if hasattr(pattern, 'url_patterns'):
+            # this is an included RegexURLResolver; recursively decorate the views
+            # contained in it
             decorate_urlpatterns(pattern.url_patterns, decorator)
 
-        if hasattr(pattern, '_callback'):
-            pattern._callback = update_wrapper(decorator(pattern.callback), pattern.callback)
+        if DJANGO_VERSION < (1, 10):
+            # Prior to Django 1.10, RegexURLPattern accepted both strings and callables as
+            # the callback parameter; `callback` is a property that consistently returns it as
+            # a callable.
+            #
+            # * if RegexURLPattern was given a string, _callback will be None, and will be
+            #   populated on the first call to the `callback` property
+            # * if RegexURLPattern was given a callable, _callback will be set to that callable,
+            #   and the `callback` property will return it
+            #
+            # In either case, we wrap the result of `callback` and write it back to `_callback`,
+            # so that future calls to `callback` will return our wrapped version.
+
+            if hasattr(pattern, '_callback'):
+                pattern._callback = update_wrapper(decorator(pattern.callback), pattern.callback)
+        else:
+            # In Django 1.10 and above, RegexURLPattern only accepts a callable as the callback
+            # parameter; this is directly accessible as the `callback` attribute.
+            if getattr(pattern, 'callback', None):
+                pattern.callback = update_wrapper(decorator(pattern.callback), pattern.callback)
 
     return urlpatterns