Provide a default __page_attribute__.

This way `browserDefault` does not die with an AttributeError but
`__call__` will do it.

src/zope/browserpage/metaconfigure.py

@@ -404,6 +404,8 @@ def _handle_for(_context, for_):
 @implementer(IBrowserPublisher)
 class simple(BrowserView):
 
+    __page_attribute__ = '__call__'
+
     def publishTraverse(self, request, name):
         if name in getattr(self, "_simple__whitelist", []):
             self.__page_attribute__ = name

src/zope/browserpage/tests/test_page.py

@@ -560,11 +560,11 @@ def testNamedViewNoPagesForNonCallable(self):
 
         view = component.getMultiAdapter((ob, self.request), name='test')
         view = removeSecurityProxy(view)
+        browser_default = view.browserDefault(self.request)
+        self.assertEqual(browser_default[1], '')
         with self.assertRaises(AttributeError) as ctx:
-            view.browserDefault(self.request)
-        self.assertEqual(
-            str(ctx.exception),
-            "'test' object has no attribute '__page_attribute__'")
+            browser_default[0]()
+        self.assertEqual(str(ctx.exception), "__call__")
 
     def testNamedViewPageViewsNoDefault(self):
         self.assertEqual(