Revision 1365 introduced partial read to zlib streams by the use of istream::readsome. As it was reported in SF# 3024551 the implementation doesn't work with the StreamCopier class on a HTTPResponse and was therefor reverted with revisions 1398 and 1410.
Besides of my belief that a stream shouldn't wait for it internal buffers to be filled, i would like to compress XML based network communication and run it through the SAX parser.
The actual problem of the Testcase provided with SF# 3024551 isn't the zlib change, it is rather that the HTTPResponse stream isn't completely read/aten and an empty line remains in the HTTPSession buffer which is processed by the HTTPHeaderStream of the following request and signals a wrong end of header.
btw. after Z_STREAM_END the zlib Streams should actually push all remaining bytes back to the source stream as they will be lost otherwise.
I don't know enough about the HTTP classes to provide a patch. I hope you will investigate the problem and add support for partial zlib stream reads again. Maybe a buffer reset in HTTPClientSession::sendRequest or HTTPClientSession::receiveResponse will fix it.