fix: add notfound() View method; auto-append trailing slash

the latter provides auto-redirect to `/widgets/` when user visits
`/widgets` for example

src/wuttaweb/views/base.py

@@ -73,6 +73,14 @@ class View:
         """
         return forms.Form(self.request, **kwargs)
 
+    def notfound(self):
+        """
+        Convenience method, to raise a HTTP 404 Not Found exception::
+
+           raise self.notfound()
+        """
+        return httpexceptions.HTTPNotFound()
+
     def redirect(self, url, **kwargs):
         """
         Convenience method to return a HTTP 302 response.

src/wuttaweb/views/common.py

@@ -53,7 +53,10 @@ class CommonView(View):
     @classmethod
     def _defaults(cls, config):
 
-        # home
+        # auto-correct URLs which require trailing slash
+        config.add_notfound_view(cls, attr='notfound', append_slash=True)
+
+        # home page
         config.add_route('home', '/')
         config.add_view(cls, attr='home',
                         route_name='home',