Can't pass JSON string as value

[ulver2812]

Hi, I'm trying to pass a JSON string as value in database.update but I get this error:
"error" : "Invalid data; couldn't parse JSON object. Are you sending a JSON object with valid key names?"

I think that the problem is in "src/Fbdatabase.cpp" at line 11:

String payload = "{\"" + key + "\":\"" + value + "\"}";

This syntax can't permit to pass a JSON string, can you improve this function or add a specific JSON function.

Thanks!