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)
```
if (jv instanceof Number) return ((Number) jv).floatValue();
                return (float) 0;
            }
        };
        GlobalDecoderMap.put(float.class, d);
        d = new Decoder() {
            @Override
            public Object decode(Object jv) {
                if (jv instanceof Number) return new Float(((Number) jv).floatValue());
                return (Float) null;
            }
        };
        GlobalDecoderMap.put(Float.class, d);
        d = new Decoder() {
            @Override
            public Object decode(Object jv) {
                if (jv instanceof Number) return ((Number) jv).doubleValue();
                return (double) 0;
            }
        };
        GlobalDecoderMap.put(double.class, d);
        d = new Decoder() {
            @Override
            public Object decode(Object jv) {
                if (jv instanceof Number) return new Double(((Number) jv).doubleValue());
                return (Double) null;
            }
        };
        GlobalDecoderMap.put(Double.class, d);
        d = new Decoder() {