Ability to specify header values to forward

[sveinnfannar]

If you rename the headers option to forwardedHeaders you can reserve the headers option to specify other headers.

For example:

server.use(include({
  forwardedHeaders: ['authorization', 'x-request-id'],
  headers: {
    'user-agent': 'my-awesome-service/1.3.37'
  }
}));