Add content length to GCP multipart complete

This commit fixes the GCP mulipart complete implementation by adding
the Content-Length header to XML requests.

According to the docs,
https://cloud.google.com/storage/docs/xml-api/post-object-complete,
this header is required in the complete POST request, but wasn't being
set previously. It seems like GCP doesn't actually validate this
header, but it's better to set it in case they validate in the future.

Additionally, GCP is strict about setting the Content-Length header on
requests with empty bodies, so we also update an empty PUT request,
https://cloud.google.com/storage/docs/xml-api/put-object-multipart,
with the header.

Author: jkosh44

object_store/src/gcp/client.rs

@@ -517,6 +517,7 @@ impl GoogleCloudStorageClient {
             // GCS doesn't allow empty multipart uploads
             let result = self
                 .request(Method::PUT, path)
+                .header(&CONTENT_LENGTH, "0")
                 .idempotent(true)
                 .do_put()
                 .await?;
@@ -540,6 +541,7 @@ impl GoogleCloudStorageClient {
         let response = self
             .client
             .request(Method::POST, &url)
+            .header(&CONTENT_LENGTH, data.len())
             .bearer_auth(&credential.bearer)
             .query(&[("uploadId", upload_id)])
             .body(data)