set (field, val) {
    if (this.headerSent || !field) return

    if (typeof field === 'string') {
      this.res.setHeader(field, val)
    } else {
      Object.keys(field).forEach(header => this.res.setHeader(header, field[header]))
    }
  }
Set header `field` to `val` or pass
an object of header fields.

Examples:

   this.set('Foo', ['bar', 'baz']);
   this.set('Accept', 'application/json');
   this.set({ Accept: 'text/plain', 'X-API-Key': 'tobi' });

@param {String|{ [k: string]: any }} field
@param {any} [val]
@api public