fix Application.addMiddleware

[artem-burlakov-ext-orion]

before:

addMiddleware(filepath) {
    const filename = path.basename(filepath);
    const middlewareName = filename.replace(path.extname(filename), '');
    log('addMiddleware', middlewareName, filepath);
    // FIXME: check uniqueness
    this.middlewares[middlewareName] = filepath;
  }

after:

addMiddleware(filepath) {
    const filename = path.basename(filepath);
    const middlewareName = filename.replace(path.extname(filename), '');
    log('addMiddleware', middlewareName, filepath);
    if (has(this.middlewares, middlewareName)) {
      throw new Error(`Middleware '${middlewareName}' already exists`);
    }
    this.middlewares[middlewareName] = filepath;
  }