middleware::Compress seems to be buffering content indefinitely

Initially reported https://github.com/lovasoa/SQLpage/issues/435

When making streaming responses and activating `middleware::Compress`, the compression seems to buffer content indefinitely, without ever flushing the streaming data when it's small enough.

Here is an example demonstrating the problem

```rust
use actix_web::{
get, middleware,
web::{self, Bytes},
App, HttpResponse, HttpServer,
};
use futures::stream::{self, StreamExt};
use std::time::Duration;
use tokio::time::sleep;

#[get("/")]
async fn index() -> HttpResponse {
let stream = stream::once(async {
let initial_content = r#"

Streaming HTML Example

"#;
Ok::<_, std::io::Error>(Bytes::from(initial_content))
})
.chain(stream::once(async {
sleep(Duration::from_secs(5)).await;
let delayed_content = r#"

"#;
Ok::<_, std::io::Error>(Bytes::from(delayed_content))
}));

HttpResponse::Ok()
.content_type("text/html; charset=utf-8")
.streaming(stream)
}

#[actix_web::main]
async fn main() -> std::io::Result<()> {
HttpServer::new(|| {
App::new()
.service(
web::scope("/compressed")
.wrap(middleware::Compress::default())
.service(index),
)
.service(index)
})
.bind("127.0.0.1:8080")?
.run()
.await
}
```

## Expected Behavior

- When visiting I should see `This content appears immediately` immediateley, and then after 5 seconds `This content appears after 5 seconds`
- When visiting http://localhost:8080/compressed/ the behavior should be the same, or as close as possible.

## Current Behavior

- When visiting http://localhost:8080/compressed/ the page stays blank for five seconds, then displays `This content appears immediately` and `This content appears after 5 seconds` at the same time.

## Possible Solution

The compression middleware should flush the writer object after a few milliseconds without new input data, to avoid indefinitely retaining data in memory on the server when it could actually already be rendered on the client.

It's possible to retain data in memory for a few milliseconds to ensure good compression ratios, but actix shouldn't keep streaming data in memory for multiple seconds.

## Context

We are trying to integrate easy native loading spinners without javascript in [SQLPage](https://sql.ophir.dev)

## Your Environment

- Actix Web Version: 4.8.0

Start with the middleware::Compress implementation and run the supplied Rust streaming example, comparing compressed and uncompressed responses. Done means the initial HTML appears promptly on the compressed endpoint while the delayed content still arrives after five seconds, without changing the documented streaming behavior.

索引モデルが issue の本文から書いたものです。