[fix] tower: "buffer full; poll_ready must be called first"

[nguquen]

When using AsyncInterceptor for tonic client through tower service, example:

let client_interceptor = ClientInterceptor::new().await?;
let auth_channel = tower::ServiceBuilder::new()
    .layer(async_interceptor(client_interceptor))
    .service(channel);
let client = ServiceClient::new(auth_channel.clone());

let response = client.get_user(Request::new(...)).await?;

it will face this error:

thread 'tokio-runtime-worker' panicked at 'buffer full; poll_ready must be called first', 
.../tower-0.4.13/src/buffer/service.rs:146:14

This fix use the same solution mentioned in here hyperium/tonic#624 (comment)

[jdahlq]

@nguquen Thanks for the PR, I appreciate you tracking down the issue and fixing it. I vaguely remember seeing this issue when I first wrote this utility, but I can't remember why I didn't end up using mem::replace. Maybe because the tests passed and my use case worked without it.

Anyway, I'll merge this and publish a new version.

[jdahlq]

@nguquen v0.11.1 has been published to Cargo