Rapidjson: add external sub-document to document
This turned out to be more simple than I thought. From GitHub (issue 436):
The easiest solution to avoid the copy is to reuse the allocator of the outer document:
rapidjson::Document jsonSubDocument(&jsonDocument.GetAllocator());wheel_.ToJson(jsonSubDocument);jsonDocument.AddMember("wheel_", jsonSubDocument, jsonDocument.GetAllocator());