message: fix writeConcern population in find_and_modify
diff: @@ -113,7 +113,7 @@ mongocxx::stdx::optional<bsoncxx::document::value> find_and_modify(
  if (!options.write_concern()->is_acknowledged() && options.collation()) {
  throw mongocxx::logic_error{mongocxx::error_code::k_invalid_parameter};
  }
  - extra.append(concatenate(options.write_concern()->to_document()));
  + extra.append(kvp("writeConcern", options.write_concern()->to_document()));
  }
  if (session_t) {
  