Honor set_decompress(false) in response streams (#2640)

open_stream_() created a decoder whatever set_decompress() said, so a response stream decoded the body when decompression was disabled, and failed with UnsupportedContentEncoding in a build without that decoder. The buffered path already checks the option; the streaming path now does too. The new test covers open_stream() and stream::Get().

Fixes #2639.

httplib.h

@@ -16020,7 +16020,7 @@ inline ClientImpl::StreamHandle ClientImpl::open_stream_(Request &req) {
 
   auto content_encoding = detail::get_combined_header_value(
       handle.response->headers, "Content-Encoding");
-  if (!content_encoding.empty()) {
+  if (decompress_ && !content_encoding.empty()) {
     // Same policy as prepare_content_receiver(): reject a coding we know about
     // but were not built with, pass an unrecognized one through as-is.
     handle.decompressor_ = detail::create_decompressor(content_encoding);