Fix tests on serializing optimizers
Fix tests on serializing optimizers

okuta/chainer,anaruse/chainer,hvy/chainer,keisuke-umezawa/chainer,wkentaro/chainer,jnishi/chainer,pfnet/chainer,wkentaro/chainer,keisuke-umezawa/chainer,niboshi/chainer,ktnyt/chainer,keisuke-umezawa/chainer,tkerola/chainer,rezoo/chainer,jnishi/chainer,chainer/chainer,okuta/chainer,okuta/chainer,hvy/chainer,jnishi/chainer,chainer/chainer,jnishi/chainer,keisuke-umezawa/chainer,aonotas/chainer,hvy/chainer,ktnyt/chainer,ktnyt/chainer,niboshi/chainer,hvy/chainer,ktnyt/chainer,ronekko/chainer,niboshi/chainer,niboshi/chainer,chainer/chainer,okuta/chainer,wkentaro/chainer,chainer/chainer,wkentaro/chainer