NEXUS-5790: Expose buffer size used to copy

This class delivers the actual content into ServletResponse
OutputStream, by performing plain copy of it.

Originally, plexus-utils IOUtil default buffer size were
used, that was 4kB. Testing showed that it's possible
to gain some performance by changing buffer sizes, so
let's expose this for tuning.

In current setup not much other choice is left, as Restlet
and Servlet hides pretty much everything from our code.