What steps will reproduce the problem?

import java.util.HashMap;
import java.util.Map;

import com.google.gson.Gson;
import com.google.gson.GsonBuilder;

public class GsonNullExample {
    public static void main(String[] args) {
        Map<String, String> map = new HashMap<String, String>();
        map.put("key", null);
        Gson gson = new GsonBuilder().serializeNulls().create();
        System.out.println(gson.toJson(map));
    }
}


What is the expected output? What do you see instead?
The output is:
   {"key":null}

I would like the option to set a custom value for null, e.g. "" :
   {"key":""}

Perhaps serializeNulls() could be overloaded to serializeNulls(String 
nullStringValue), to allow custom String instead of the word "null".

What version of the product are you using? On what operating system?
gson 2.1 on Mac OSx using Eclipse.

Please provide any additional information below.