wallet-http: TransactionOptions minor refactor

[nodech]

This allows tx options to have different defaults.

e.g.

  const options = new TransactionOptions();
  options.blocks = 10;
  options.fromValidator(valid);

This will override options.blocks only when validator has it.

[coveralls]

coverage: 71.655% (+0.02%) from 71.638%
when pulling a30427f on nodech:transaction-http-validator
into 8f42b5d on handshake-org:master.