ASGI HTTP protocol 2.4: send() may raise OSError

[vytas7]

In the ASGI HTTP & WebSocket protocol version 2.4, calling send() on a closed connection should raise an OSError.

...but I haven't really been able to reproduce this in real servers (Uvicorn, Hypercorn).

Also, considering how the flow usually looks like, this is more important for WebSockets, which will be addressed separately in #2292.

[vytas7]

This gets probably even less urgent now that Uvicorn has backpedaled on this change, and reverted back to the 2.3 version of the spec: encode/uvicorn#2276.