message: use merge instead of assign for updating class names
diff: @@ -15,6 +15,8 @@ import {
  getPath,
  makeEventsArray,
  makeDelayObject,
  + merge,
  + isObject
  } from './utils';
  import Generator from './generator';
  import Validator from './validator';
  @@ -210,7 +212,7 @@ export default class Field {
  this.model = options.model || this.model;
  this.listen = options.listen !== undefined ? options.listen : this.listen;
  this.classes = (options.classes || this.classes || false) && !this.component;
  - this.classNames = options.classNames || this.classNames || DEFAULT_OPTIONS.classNames;
  + this.classNames = isObject(options.classNames) ? merge(options.classNames, DEFAULT_OPTIONS.classNames) : this.classNames;
  this.getter = isCallable(options.getter) ? options.getter : this.getter;
  this._alias = options.alias || this._alias;
  this.events = (options.events) ? makeEventsArray(options.events) : this.events;
  