Fix multipart uploads on Minio (#2731)

The official Minio SDK uses "uploads=" as the URL when it initiates a
multipart upload instead of "uploads". This affects the AWSV4 signature
and causes object_store to fail a signature check when initiating the
upload to Minio.

It's possible that this contradicts the AWS S3 API docs:
https://docs.aws.amazon.com/AmazonS3/latest/API/API_CreateMultipartUpload.html#API_CreateMultipartUpload_RequestSyntax
and we need to instead keep the URL as `?uploads` and
change the URL that goes into the signature instead.

object_store/src/aws/client.rs

@@ -411,7 +411,7 @@ impl S3Client {
     pub async fn create_multipart(&self, location: &Path) -> Result<MultipartId> {
         let credential = self.get_credential().await?;
         let url = format!(
-            "{}/{}/{}?uploads",
+            "{}/{}/{}?uploads=",
             self.config.endpoint,
             self.config.bucket,
             encode_path(location)