@UnstableApi
    @Override
    default HttpRequest unsplit() {
        final CompletableFuture<HttpHeaders> trailersFuture = trailers().thenApply(trailers -> {
            if (trailers.isEmpty()) {
                // An empty trailers means that the request does not have trailers.
                return null;
            } else {
                return trailers;
            }
        });
        return HttpRequest.of(headers(), body(), trailersFuture);
    }
Combines the split {@link #headers()}, {@link #body()}, {@link #trailers()} into
a single {@link HttpRequest}.

<p>Note that this method can only be used before subscribing to {@link #body()}.
core/src/main/java/com/linecorp/armeria/common/SplitHttpRequest.java