Cannot Access Trailer-Headers in Chunked transfer-encoding

This issue was previously identified and fixed here for HttpClient version 5.0. To my knowledge this issue has been present since version 4.3 when wrappers were added for response entities and content streams and access to the underlying ChunkedInputStream was restricted.

My proposal is to backport the fix from HTTPCLIENT-1992 to 4.5.x and add a getTrailers() method to ResponseEntityProxy.