Make PutObject a nop for an object which ends with "/" and size is '0' (#3603)

This helps majority of S3 compatible applications while not returning
an error upon directory create request.

Fixes #2965

cmd/xl-v1-object.go

@@ -440,6 +440,12 @@ func renameObject(disks []StorageAPI, srcBucket, srcObject, dstBucket, dstObject
 // writes `xl.json` which carries the necessary metadata for future
 // object operations.
 func (xl xlObjects) PutObject(bucket string, object string, size int64, data io.Reader, metadata map[string]string, sha256sum string) (objInfo ObjectInfo, err error) {
+	// This is a special case with size as '0' and object ends with
+	// a slash separator, we treat it like a valid operation and
+	// return success.
+	if isObjectDir(object, size) {
+		return dirObjectInfo(bucket, object, size, metadata), nil
+	}
 	if err = checkPutObjectArgs(bucket, object, xl); err != nil {
 		return ObjectInfo{}, err
 	}