[#431] Fix ETag value

ETag is a string with SHA256 of NeoFS object payload.
Hash is taken from object header by neofs-sdk-go getter.
Checksum type in neofs-sdk-go has changes `String()`
output from `<hash>` to `SHA256:<hash>` in latest update.

S3 gateway should not be relied on unstable string format
implementations and use raw value.

Signed-off-by: Alex Vanin <alexey@nspcc.ru>

api/layer/object.go

@@ -2,6 +2,7 @@ package layer
 
 import (
 	"context"
+	"encoding/hex"
 	"errors"
 	"fmt"
 	"io"
@@ -260,7 +261,7 @@ func (n *layer) PutObject(ctx context.Context, p *PutObjectParams) (*data.Object
 		CreationEpoch: meta.CreationEpoch(),
 		Headers:       p.Header,
 		ContentType:   p.Header[api.ContentType],
-		HashSum:       payloadChecksum.String(),
+		HashSum:       hex.EncodeToString(payloadChecksum.Value()),
 	}, nil
 }