Do not handle Close during file transfer as a failed requirement.

downloader.go

@@ -146,10 +146,6 @@ func (client *Downloader) Close() error {
 		return nil
 	}
 
-	if client.state.Type == TransmittingFile {
-		return ErrInvalidState
-	}
-
 	client.dataChannel.Close()
 	return nil
 }

uploader.go

@@ -106,10 +106,6 @@ func (server *Uploader) Close() error {
 		return nil
 	}
 
-	if server.state.Type == TransmittingFile {
-		return ErrInvalidState
-	}
-
 	server.dataChannel.Close()
 	return nil
 }