Fix for writes from Apache Spark. (#4074)

- Due to usage of amazon SDK, spark expects md5sum of empty string to be
  returned when it does PUT on a directory.
- The fix returns md5sum of a empty string for the above mentioned case.
- This fixes the issue of Apache Spark not being able to write into Minio.

cmd/object-api-common.go

@@ -34,6 +34,9 @@ const (
 
 	// Buckets meta prefix.
 	bucketMetaPrefix = "buckets"
+
+	// Md5Sum of empty string.
+	emptyStrMd5Sum = "d41d8cd98f00b204e9800998ecf8427e"
 )
 
 // Global object layer mutex, used for safely updating object layer.
@@ -67,6 +70,10 @@ func dirObjectInfo(bucket, object string, size int64, metadata map[string]string
 	// return success.
 	md5Sum := metadata["md5Sum"]
 	delete(metadata, "md5Sum")
+	if md5Sum == "" {
+		md5Sum = emptyStrMd5Sum
+	}
+
 	return ObjectInfo{
 		Bucket:      bucket,
 		Name:        object,