Add content length and date headers to CORS response

Load balancers such as [ELB](http://aws.amazon.com/elasticloadbalancing/) will return a `502` to a client's preflight request due to the omission of date and content-length headers in Netty.

Am not sure this is the best place to put this, but some way of appending minimal standard response headers would be nice... maybe as a flag in `HttpServerCodec`?