Bug of fileBlacklist shadow merge

[yinsang]

When using the fileBlacklist of options, it will cover the default option([/.map/]). Wish: fileBlacklist:[/userIgnore\.js/, /\.map/] Actually fileBlacklist: [/userIgnore\.js/] I wonder if it‘s a Bug or design of this shadow merge?

class PreloadPlugin {
  constructor(options) {
    this.options = Object.assign({}, defaultOptions, options);
  }
}