Builder::finish() does not 'close' inner Writer

`Builder::finish()` doesn't seem to flush the state of the inner writer. this means the following snippet doesn't work-

```rust
use async_compression::futures::write::GzipEncoder;
use async_std::{fs::File, path::Path};
use async_tar::Builder;
use futures::io::AsyncWrite;

async fn write_archive(writer: W, src_directory: impl AsRef) -> std::io::Result<()>
where
W: AsyncWrite + Unpin + Send + Sync,
{
let mut archive_builder = Builder::new(writer);
archive_builder.append_dir_all("", src_directory).await?;
archive_builder.finish().await
}

#[async_std::main]
async fn main() {
let file = File::create("foo.tar.gz").await.unwrap();
let encoder = GzipEncoder::new(file);
write_archive(encoder, "sample-directory").await.unwrap();
}
```

but this snippet *does* work

```rust
use async_compression::futures::write::GzipEncoder;
use async_std::{fs::File, path::Path};
use async_tar::Builder;
use futures::io::{AsyncWrite, AsyncWriteExt};

async fn write_archive(writer: W, src_directory: impl AsRef) -> std::io::Result<()>
where
W: AsyncWrite + Unpin + Send + Sync,
{
let mut archive_builder = Builder::new(writer);
archive_builder.append_dir_all("", src_directory).await?;
archive_builder.finish().await?;

archive_builder.into_inner().await?.close().await
}

#[async_std::main]
async fn main() {
let file = File::create("foo.tar.gz").await.unwrap();
let encoder = GzipEncoder::new(file);
write_archive(encoder, "docker-context").await.unwrap();
}
```

that's more than a little surprising.

`Builder::finish()` should flush the inner writer, or at the very least, this behaviour should be clearly documented