Custom Format Support - variable binary field within first 127 fields

[arajput]

First thanks for this awesome library. I am using this for some IFSF messages encode/decode at client side application. i have some custom requirement t support subfields at 48 position. It has bitmap + 48.1, 48.2, 48.3 ... very much same as 127.25
i can not use "LLLVAR ans" because it is at host side the field content is treated as binary similar to fixed binary field.

here is test case attached for the above requirement.

import test from 'ava';
import iso8583 from '../lib/8583.js';

/*
Support custom iso 8583 formats
Support Case: Binary var field encode decode
*/
test('iso8583 Ext - binary var field must be encoded', t => {
  const format = {
    2: {
      ContentType: 'b',
      Label: 'F1',
      LenType: 'llvar',
      MaxLen: 99
    },
        
  };
  const data = {
    0: '0800',
    '2': '1004000000000000303030303030303030343048' 
    // #2 is prepard by encodng subfields - 8bytes bitmap followed by subfields data
  };
  let isopack = new iso8583(data, format);
  let buff = isopack.getRawMessage();
  const iso1 = new iso8583(undefined, format).getIsoJSON(buff, {lenHeader: false});
  t.deepEqual(iso1, data);
  let isopack1 = new iso8583(iso1);
  t.is(isopack1.Msg['2'], '1004000000000000303030303030303030343048');
});

[arajput]

Please see my observation in the code.

[arajput]

i have following commit that has helped me for my case https://github.com/arajput/iso_8583/commit/54dc08b688cc3621c660c355cf121c40f0acac75

[zemuldo]

I will check this over the weekend. I am happy to review a pull request if you send it.