fix backprop_test.cc to pass
fix backprop_test.cc to pass

pfnet/chainer,keisuke-umezawa/chainer,chainer/chainer,keisuke-umezawa/chainer,okuta/chainer,jnishi/chainer,jnishi/chainer,wkentaro/chainer,chainer/chainer,wkentaro/chainer,keisuke-umezawa/chainer,okuta/chainer,niboshi/chainer,hvy/chainer,niboshi/chainer,hvy/chainer,chainer/chainer,okuta/chainer,ktnyt/chainer,wkentaro/chainer,jnishi/chainer,wkentaro/chainer,okuta/chainer,niboshi/chainer,jnishi/chainer,keisuke-umezawa/chainer,ktnyt/chainer,ktnyt/chainer,hvy/chainer,tkerola/chainer,ktnyt/chainer,hvy/chainer,chainer/chainer,niboshi/chainer