Fix: File Transport not flush buffers to file

[adoyle-h]

fix #228 #1504

Test Case:

// ./test/helpers/scripts/logger-end-and-process-exit.js
'use strict';

const path = require('path');
const winston = require('../../../lib/winston');

var logger = winston.createLogger({
  transports: [
    new winston.transports.File({
      filename: path.join(__dirname, '..', '..', 'fixtures', 'logs', 'logger-end-and-process-exit.log')
    })
  ]
});

logger.on('finish', () => {
  process.exit(0);
});

logger.info('CHILL WINSTON!', { seriously: true });

logger.end();

[rheh]

Potentially violates the @returns {undefined} docs as cb could return anything?

[rheh]

Arrow function not required?

this._dest.once('finish', cb);

[rheh]

Arrow function not required?

this.close(this._endStream);

[adoyle-h]

@rheh Fixed. Please review again.