PUT /api/tasks/:id implementation

put.js

api.put('/api/tasks/:id', (req, res, next) => {
  const description = req.body.description
  const completed = req.body.completed

  let processor = null

  if (description) {
    processor = Processors.update('description', description)
  }
  if (typeof(completed) !== 'undefined') {
    const compProcessor = Processors.update('completed', completed)
    processor = processor ? processor.andThen(compProcessor) : compProcessor
  }

  tasks.invoke(req.params.id, processor)
      .then(result => res.send(result))
      .catch(err => next(err))
})