fix from dubbo:master, 2.5.7 : GenericJSONConverter主要是用在日志部分输出，所以当我们输出locale对象时就发生stack溢出。
```
java.lang.StackOverflowError
	at com.alibaba.dubbo.common.json.JSONWriter.valueString(JSONWriter.java:189)
	at com.alibaba.dubbo.common.json.GenericJSONConverter$7.encode(GenericJSONConverter.java:100)
	at com.alibaba.dubbo.common.json.GenericJSONConverter.writeValue(GenericJSONConverter.java:338)
	at com.alibaba.dubbo.common.json.GenericJSONConverter.writeValue(GenericJSONConverter.java:347)
	at com.alibaba.dubbo.common.json.GenericJSONConverter.writeValue(GenericJSONConverter.java:395)
	at com.alibaba.dubbo.common.json.GenericJSONConverter.writeValue(GenericJSONConverter.java:347)
	at com.alibaba.dubbo.common.json.GenericJSONConverter.writeValue(GenericJSONConverter.java:395)
```
Decoder d = new Decoder() {
            @Override
            public Object decode(Object jv) {
                return jv.toString();
            }
        };
        GlobalDecoderMap.put(String.class, d);
        d = new Decoder() {
            @Override
            public Object decode(Object jv) {
                if (jv instanceof Boolean) return ((Boolean) jv).booleanValue();
                return false;
            }
        };
        GlobalDecoderMap.put(boolean.class, d);
        d = new Decoder() {
            @Override
            public Object decode(Object jv) {
                if (jv instanceof Boolean) return (Boolean) jv;
                return (Boolean) null;
            }
        };
        GlobalDecoderMap.put(Boolean.class, d);
        d = new Decoder() {
            @Override
            public Object decode(Object jv) {
                if (jv instanceof String && ((String) jv).length() > 0) return ((String) jv).charAt(0);
                return (char) 0;
            }
        };