Filter Expires header from user metadata (#7269)

Instead save it as a struct field in ObjectInfo as it is
a standard HTTP header - Fixes minio/mc#2690

cmd/gateway/gcs/gateway-gcs.go

@@ -797,12 +797,22 @@ func fromGCSAttrsToObjectInfo(attrs *storage.ObjectAttrs) minio.ObjectInfo {
 	// All google cloud storage objects have a CRC32c hash, whereas composite objects may not have a MD5 hash
 	// Refer https://cloud.google.com/storage/docs/hashes-etags. Use CRC32C for ETag
 	metadata := make(map[string]string)
+	var (
+		expiry time.Time
+		e      error
+	)
 	for k, v := range attrs.Metadata {
 		k = http.CanonicalHeaderKey(k)
 		// Translate the GCS custom metadata prefix
 		if strings.HasPrefix(k, "X-Goog-Meta-") {
 			k = strings.Replace(k, "X-Goog-Meta-", "X-Amz-Meta-", 1)
 		}
+		if k == "Expires" {
+			if expiry, e = time.Parse(http.TimeFormat, v); e == nil {
+				expiry = expiry.UTC()
+			}
+			continue
+		}
 		metadata[k] = v
 	}
 	if attrs.ContentType != "" {
@@ -829,6 +839,7 @@ func fromGCSAttrsToObjectInfo(attrs *storage.ObjectAttrs) minio.ObjectInfo {
 		UserDefined:     metadata,
 		ContentType:     attrs.ContentType,
 		ContentEncoding: attrs.ContentEncoding,
+		Expires:         expiry,
 	}
 }