code: function _update(model, _id, payload, Log) {
    let request = { params: { _id: _id }, payload: payload }
    return _updateHandler(model, _id, request, Log)
  }
docs: Update function exposed as a mongoose wrapper.
  @param model: A mongoose model.
  @param _id: The document id.
  @param payload: Data used to update the model document.
  @param Log: A logging object.
  @returns {object} A promise for the resulting model document.
  @private