When testing the Blockchain class, make sure the addBlock method correctly adds previousHash to the added block

[azangru]

The addBlock method of the Blockchain class should pass the correct previousHash to the newly added block. This is not currently checked in the tests for the Blockchain class. So when I made the following typo:

  addBlock(transactions) {
    const previousHash = this.getHeadBlock().previousHash; // instead of this.getHeadBlock().hash;
    const newBlock = new Block(transactions, previousHash);
    this.blocks.push(newBlock);
  }

the tests didn’t catch that (although later tests did).

[therobinkim]

Fixed by #112