swift, s3, dropbox: fix metadata read on Update()

This was causing changed files to be marked as corrupted on upload

s3/s3.go

@@ -518,6 +518,7 @@ func (o *FsObjectS3) Update(in io.Reader, modTime time.Time, size int64) error {
 		return err
 	}
 	// Read the metadata from the newly created object
+	o.meta = nil // wipe old metadata
 	err = o.readMetaData()
 	return err
 }