🐛 Fix root_path behavior

[richin13]

Description

I came across a bug with setting the root_path value directly into the FastAPI app. The docs state:

Passing the root_path to FastAPI would be the equivalent of passing the --root-path command line option to Uvicorn or Hypercorn.

But that wasn't the case since providing the --root-path option to Uvicorn or Hypercorn makes it so that the scope's path and raw_path contain the root_path. The behavior in FastAPI was to patch the scope's root_path value only, leaving the path and raw_path unchanged.

When running behind a proxy, this would cause the 307 redirect on missing trailing slashes to set the Location header to the path without the root_path value, resulting in a 404 error.

[insistence]

@tiangolo Excuse me, would it be convenient for you to prioritize this PR?