Update MainService.Contracts.cs

[chenzhitong]

close #768

[ProDog]

Put it with the original code?

            if (!NoWallet() && sender != null)
            {
                if (signerAccounts == null)
                    signerAccounts = new UInt160[1] { sender };
                else if (signerAccounts.Contains(sender) && signerAccounts[0] != sender)
                {
                    var signersList = signerAccounts.ToList();
                    signersList.Remove(sender);
                    signerAccounts = signersList.Prepend(sender).ToArray();
                }
                else if (!signerAccounts.Contains(sender))
                {
                    signerAccounts = signerAccounts.Prepend(sender).ToArray();
                }
                signers = signerAccounts.Select(p => new Signer() { Account = p, Scopes = WitnessScope.CalledByEntry }).ToArray();
            }

[shargon]

tested?

[ProDog]

Tested OK.