code: function (options) {
          if (!$this._multipart) {
            $this.options.multipart = []
          }
  
          if (is(options).a(Object)) {
            if (options['content-type']) {
              var type = Unirest.type(options['content-type'], true)
              if (type) options.body = Unirest.Response.parse(options.body)
            } else {
              if (is(options.body).a(Object)) {
                options.body = Unirest.serializers.json(options.body)
              }
            }
  
            $this.options.multipart.push(options)
          } else {
            $this.options.multipart.push({
              body: options
            })
          }
  
          return $this
        }
docs: Takes multipart options and places them on `options.multipart` array.
  Transforms body when an `Object` or _content-type_ is present.
  
  Example:
  
  Unirest.get('http://google.com').part({
  'content-type': 'application/json',
  body: {
  phrase: 'Hello'
  }
  }).part({
  'content-type': 'application/json',
  body: {
  phrase: 'World'
  }
  }).end(function (response) {})
  
  @param  {Object|String} options When an Object, headers should be placed directly on the object,
  not under a child property.
  @return {Object}