RestClientEmscripten: Preserve query parameters for sub/unsub

Make sure LongPollingIdx is appended to existing query params, not
replacing them.

src/client/include/RestClientEmscripten.hpp

@@ -273,7 +273,7 @@ class RestClient : public ClientBase {
  }
 
  void startSubscription(Command &&cmd) {
- auto uri = opencmw::URI<>::factory(cmd.topic).queryParam("LongPollingIdx=Next").build();
+ auto uri = opencmw::URI<>::factory(cmd.topic).addQueryParameter("LongPollingIdx", "Next").build();
  cmd.topic = uri;
 
  auto payload = std::make_unique<detail::SubscriptionPayload>(std::move(cmd), _mimeType);
@@ -288,7 +288,7 @@ class RestClient : public ClientBase {
  // void get(...)
  // void set(...)
  // instead of going through a fake generic request(...)?
- auto uri = opencmw::URI<>::factory(cmd.topic).queryParam("LongPollingIdx=Next").build();
+ auto uri = opencmw::URI<>::factory(cmd.topic).addQueryParameter("LongPollingIdx", "Next").build();
  auto payloadIt = std::find_if(detail::subscriptionPayloads.begin(), detail::subscriptionPayloads.end(),
  [&](const auto &ptr) {
  return ptr->command.topic == uri;