[#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/util.go

@@ -2,6 +2,7 @@ package layer
 
 import (
 	"context"
+	"encoding/hex"
 	"fmt"
 	"os"
 	"strconv"
@@ -126,7 +127,7 @@ func objectInfoFromMeta(bkt *data.BucketInfo, meta *object.Object, prefix, delim
 		Headers:       userHeaders,
 		Owner:         meta.OwnerID(),
 		Size:          size,
-		HashSum:       payloadChecksum.String(),
+		HashSum:       hex.EncodeToString(payloadChecksum.Value()),
 	}
 }