Expose netSocket member on HttpClientResponse and DefaultHttpClientResponse

Make groovy HttpClientResponse and DefaultHttpClientResponse classes consistent with their core Java versions by exposing the netSocket member. 

While the use-cases for needing visibility of this member are limited, I've run into one when working with the Twitter streaming API (want to be able to kill the response from the client side at will when I've got all the data I need from the stream - closing the netsocket connection would appear to be the least bad approach to this).

This includes a test (ported from the Java codebase) for the netSocket member.