Fix toString of draughts board (#670)

Author: ishii-norimi

lib/rl/draughts.js

@@ -284,9 +284,9 @@ class DraughtsBoard {
 				if (j > 0) {
 					buf += ' '
 				}
-				if (this._board[i][j] === RED) {
+				if (this._board[i][j] & RED) {
 					buf += 'x'
-				} else if (this._board[i][j] === WHITE) {
+				} else if (this._board[i][j] & WHITE) {
 					buf += 'o'
 				} else {
 					buf += '-'
@@ -298,19 +298,13 @@ class DraughtsBoard {
 	}
 
 	nextTurn(turn) {
-		if (turn === WHITE) {
-			return RED
-		} else {
-			return WHITE
-		}
+		return turn === WHITE ? RED : WHITE
 	}
 
 	copy() {
 		const cp = new DraughtsBoard(this._size, this._evaluator)
 		for (let i = 0; i < this._size[0]; i++) {
-			for (let j = 0; j < this._size[1]; j++) {
-				cp._board[i][j] = this._board[i][j]
-			}
+			cp._board[i] = this._board[i].concat()
 		}
 		return cp
 	}