Reward transaction does not get signed

[ozshm]

Hello, great job on this project!

EDIT: Nevermind. A transaction is valid when a null sender is set.

Please, I would like to draw your attention to the Blockchain.minePendingTransactions() method (here):

minePendingTransactions(miningRewardAddress) {
    const rewardTx = new Transaction(null, miningRewardAddress, this.miningReward);
    this.pendingTransactions.push(rewardTx);

    const block = new Block(Date.now(), this.pendingTransactions, this.getLatestBlock().hash);
    block.mineBlock(this.difficulty);

    debug('Block successfully mined!');
    this.chain.push(block);
    this.pendingTransactions = []; 
}

The rewardTx is inserted into the pending transactions array without getting signed by the sender which causes the transaction to be invalid (when verified using the Transaction.isValid() method.)