Add x-amz-expiration header in some S3 responses (#9667)

x-amz-expiration is described in the S3 specification as a header which
indicates if the object in question will expire any time in the future.

cmd/http/headers.go

@@ -80,6 +80,9 @@ const (
 	// Multipart parts count
 	AmzMpPartsCount = "x-amz-mp-parts-count"
 
+	// Object date/time of expiration
+	AmzExpiration = "x-amz-expiration"
+
 	// Dummy putBucketACL
 	AmzACL = "x-amz-acl"