Ensure multiple shutdown attempts yield

[cmleinz]

Currently if the state is set to disconnect, and the user hits poll_shutdown, the function will never yield.

let mut connection = StubbornTcpStream::connect(ADDR).await.unwrap();
let _ = connection.shutdown().await;
let _ = connection.shutdown().await; // Second attempt never yields

My proposed solution (similar to how tokio operates) is to just error in the event a closed stream is told to shutdown again.

[craftytrickster]

Thanks for catching this. Code looks correct, just want to manually run some checks and verify before merging after I'm done with work.

[craftytrickster]

Thank you for the work, please address these comments and then we can merge.

[craftytrickster]

Thanks for making the test changes, looks good

[craftytrickster]

Thank you for your help!