Adding support for specifying preflight response headers.

Motivation:

An intermediary like a load balancer might require that a Cross Origin
Resource Sharing (CORS) preflight request have certain headers set.
As a concrete example the Elastic Load Balancer (ELB) requires the
'Date' and 'Content-Length' header to be set or it will fail with a 502
error code.

This works is an enhancement of https://github.com/netty/netty/pull/2290

Modifications:

CorsConfig has been extended to make additional HTTP response headers
configurable for preflight responses. Since some headers, like the
'Date' header need to be generated each time, m0wfo suggested using a
Callable.

Result:

By default, the 'Date' and 'Content-Lenght' headers will be sent in a
preflight response. This can be overriden and users can specify
any headers that might be required by different intermediaries.