message: fix(core): don't add extra `0x` prefix when formatting for offline vault
  The version of `@ethereumjs/tx` which was recently upgraded to no longer needs
  an explicit `0x` prefix added to the `to` field on transaction objects.
  A merge race allowed this to get into master without failing any tests.
  Ticket:
diff: @@ -758,7 +758,7 @@ export class Eth extends BaseCoin {
  */
  formatForOfflineVault(
  txInfo: UnformattedTxInfo,
  - ethTx: any,
  + ethTx: EthTxLib.Transaction,
  userKey: string,
  backupKey: string,
  gasPrice: Buffer,
  @@ -767,6 +767,9 @@ export class Eth extends BaseCoin {
  ): Bluebird<OfflineVaultTxInfo> {
  const self = this;
  return co<OfflineVaultTxInfo>(function* (): any {
  + if (!ethTx.to) {
  + throw new Error('Eth tx must have a `to` address');
  + }
  const backupHDNode = bip32.fromBase58(backupKey);
  const backupSigningKey = backupHDNode.publicKey;
  const response: OfflineVaultTxInfo = {
  @@ -777,7 +780,7 @@ export class Eth extends BaseCoin {
  gasPrice: optionalDeps.ethUtil.bufferToInt(gasPrice).toFixed(),
  gasLimit,
  recipients: [txInfo.recipient],
  - walletContractAddress: '0x' + ethTx.to.toString('hex'),
  + walletContractAddress: ethTx.to.toString(),
  amount: txInfo.recipient.amount,
  backupKeyNonce: yield self.getAddressNonce(
  `0x${optionalDeps.ethUtil.publicToAddress(backupSigningKey, true).toString('hex')}`
  