make ProxyToServerConnection accessible for filters

Hi there, 
I face the problem that I need to validate the body of a response together with the ChainedProxy used for the request. As far as I can see the only way to access the body is to use a HttpFilter (and a response buffer limit) and the only way to access the used ChainedProxy is through an ActivityTracker, but there seems to be no way to access both at the same time. 

It is quite easy to hand down the ProxyToServerConnection to the HttpFilter, but it breaks the API. Is this something you would consider merging? Or is there a better way to access ChainedProxy?

Cheers, Lars