Fix S3 API multipart upload on Windows (#4666)

The file paths were being having '/' changed to '\' by filepath.Dir()
resulting in a file being created with '\' separators, but when trying
to read the files, the same wasn't happening.

Co-authored-by: Hendrik Appel <happel@europe.altair.com>

weed/s3api/filer_multipart.go

@@ -122,7 +122,7 @@ func (s3a *S3ApiServer) completeMultipartUpload(input *s3.CompleteMultipartUploa
 	}
 
 	entryName := filepath.Base(*input.Key)
-	dirName := filepath.Dir(*input.Key)
+	dirName := filepath.ToSlash(filepath.Dir(*input.Key))
 	if dirName == "." {
 		dirName = ""
 	}