Default headers not signed for CompleteMultipartUpload

**Describe the bug**

When doing a multi-part upload with custom default headers, I got error
```
AccessDeniedThere were headers present in the request which were not signedx-amz-source-arn, x-amz-source-account
```

**To Reproduce**

```
use object_store::ClientOptions;
use object_store::HeaderMap;
use object_store::ObjectStoreExt;
use object_store::aws::AmazonS3Builder;
use object_store::path::Path;

#[tokio::main]
async fn main() {
let mut headers = HeaderMap::new();
headers.insert("x-amz-source-account", "123456789111".parse().unwrap());
headers.insert(
"x-amz-source-arn",
"arn:aws:s3:::source-bucket"
.parse()
.unwrap(),
);

let client = AmazonS3Builder::from_env()
.with_bucket_name("bucket-name")
.with_region("us-east-1")
.with_client_options(ClientOptions::new().with_default_headers(headers))
.build()
.unwrap();

let mut upload = client.put_multipart(&Path::from("test-mpu")).await.unwrap();
let res = upload.complete().await.unwrap();
println!("Upload complete: {:?}", res);
}

```

**Expected behavior**

Headers are signed. No error.

**Additional context**

Start from the AmazonS3Builder and put_multipart(...).complete() flow shown in the reproduction, then trace how the CompleteMultipartUpload request is signed. Reproduce the failure with the provided default headers and verify that both x-amz-source-arn and x-amz-source-account are included in the signature and the completion succeeds.

Written by the indexing model from the issue text.