When forking chain, Tx nonce should be set

[earlbread]

After #307, Tx nonce is not set except signersToStrip addresses.
Reproduce scenario is below:

var privateKey1 = new PrivateKey();
var privateKey2 = new PrivateKey();

var address1 = privateKey1.PublicKey.ToAddress();
var address2 = privateKey2.PublicKey.ToAddress();

_blockChain.MineBlock(address1);

_blockChain.MakeTransaction(privateKey1, new[] { new DumbAction() });
Block<DumbAction> branchPoint = _blockChain.MineBlock(address1);

_blockChain.MakeTransaction(privateKey2, new[] { new DumbAction() });
_blockChain.MineBlock(address2);

BlockChain<DumbAction> forked = _blockChain.Fork(branchPoint.Hash);
Assert.Equal(1, forked.GetNextTxNonce(address1));

[dahlia]

I'm going to fix this.