Set the zoom level of the map.
@param {Object} state Current state.
@param {Object} action Action to handle.

@returns {Object} The new state.
function setZoom(state, action) {
  let zoom = Math.min(DEFAULT_ZOOM.MAX, action.zoom);
  zoom = Math.max(DEFAULT_ZOOM.MIN, zoom);
  return Object.assign({}, state, {zoom});
}