Backend request URI is not computed properly

I have first encountered this issue when trying to invoke an API with base path set to `/api`, and noticed that the gateway is not properly proxying my request to the configured backend.

The request is hitting the gateway with something similar to `apigateway.com/api/abcdef/api` and forwarding to the backend with `backend.com/abcdef/api`, where it should just proxy to `backend.com/`.

Digging into it further, I have noticed that we cannot set a base path that matches any substring of `/api/`. For example, `apigateway.com/api/abcdef/c` will proxy to `backend.com/def/c`.