fix move function

feedback from OwlTest:

1. [-7.0 pts] merge([2, 4, 2, 4]) returned [4, 4, 4, 0] instead of [2,
4, 2, 4]

2. [-30.0 pts]
game = TwentyFortyEight(4, 4)
game.move(UP)

Input board:
[[2, 0, 0, 0],
 [0, 2, 0, 0],
 [0, 0, 2, 0],
 [0, 0, 0, 2]]

Expected board:
[[2, 2, 2, 2],
 [0, 0, 0, 0],
 [0, 0, 0, 0],
 [0, 0, 0, 0]]
 PLUS A NEW TILE

Actual board:
[[2, 2, 2, 2],
 [0, 0, 0, 0],
 [0, 0, 0, 0],
 [0, 0, 0, 0]]

3. [-20.0 pts] 10 style warnings found (maximum allowed: 10 style
warnings)