Handle root_path in URLRouter

If we have a scope["root_path"], raise if it does not prefix
scope["path"], and strip it from scope["path"] if it does. Do this
only if we're in an outermost URLRouter.

Signed-off-by: Alejandro R. Sedeño <asedeno@mit.edu>
Signed-off-by: Alejandro R Sedeño <asedeno@google.com>

channels/routing.py

@@ -100,6 +100,15 @@ async def __call__(self, scope, receive, send):
         path = scope.get("path_remaining", scope.get("path", None))
         if path is None:
             raise ValueError("No 'path' key in connection scope, cannot route URLs")
+
+        if "path_remaining" not in scope:
+            # We are the outermost URLRouter, so handle root_path if present.
+            root_path = scope.get("root_path", "")
+            if root_path and not path.startswith(root_path):
+                # If root_path is present, path must start with it.
+                raise ValueError("No route found for path %r." % path)
+            path = path[len(root_path) :]
+
         # Remove leading / to match Django's handling
         path = path.lstrip("/")
         # Run through the routes we have until one matches