performance improvement for ServerResponseWriter

Using a static cache of standard MediaTypes speeds up calls to getResponseMediaType() when the MediaType accepted by the request is found in the cache, by avoiding creation of new HashMaps and MediaType instances where possible.

This will not change behavior when the required MediaType is not found in the cache.

A copy of the modified class is attached - changes are marked with
// Liberty change
flags, as we have made these changes in the overlay we use in WebSphere Liberty.

Note: with a simple app, this change increased throughput by ~5%.