End the stream at the header section for HEAD, 204 and 304 in open_stream (#2636)

ClientImpl::open_stream built the body reader from Content-Length and Transfer-Encoding whatever the method or status, so a stream on a response that RFC 9112 6.3 ends at its header section was read on: with a keep-alive server read() blocked until the read timeout, and when the server closed it reported a read error for a complete response. stream::Head() against cpp-httplib's own server, which puts the GET body's Content-Length on a HEAD response, hit it every time.

open_stream_() now returns with the body reader at its end for a response to HEAD and for a 204 or 304, as the buffered path skips the body for them. Content-Encoding is skipped too, so a coding the build lacks no longer fails such a stream.

httplib.h

@@ -15939,20 +15939,25 @@ inline ClientImpl::StreamHandle ClientImpl::open_stream_(Request &req) {
     return handle;
   }
 
-  // Same framing check as ClientImpl::process_request(). A HEAD or bodyless
-  // (204/304) response legitimately carries framing headers with no body.
-  if (req.method != "HEAD" &&
-      handle.response->status != StatusCode::NoContent_204 &&
-      handle.response->status != StatusCode::NotModified_304 &&
-      detail::has_conflicting_content_length(handle.response->headers)) {
+  handle.body_reader_.stream = handle.stream_;
+  handle.body_reader_.payload_max_length = payload_max_length_;
+
+  // RFC 9112 6.3: a response to HEAD, and a 204 or 304 response, ends with its
+  // header section, so none of its framing or coding headers apply.
+  if (req.method == "HEAD" ||
+      handle.response->status == StatusCode::NoContent_204 ||
+      handle.response->status == StatusCode::NotModified_304) {
+    handle.body_reader_.eof = true;
+    return handle;
+  }
+
+  // Same framing check as ClientImpl::process_request()
+  if (detail::has_conflicting_content_length(handle.response->headers)) {
     handle.error = Error::Read;
     handle.response.reset();
     return handle;
   }
 
-  handle.body_reader_.stream = handle.stream_;
-  handle.body_reader_.payload_max_length = payload_max_length_;
-
   if (handle.response->has_header("Content-Length")) {
     bool is_invalid = false;
     auto content_length = detail::get_header_value_u64(