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)
```
jb.valueDouble(((Number) obj).doubleValue());
            }
        };
        GlobalEncoderMap.put(double.class, e);
        GlobalEncoderMap.put(Double.class, e);
        GlobalEncoderMap.put(BigDecimal.class, e);
        e = new Encoder() {
            @Override
            public void encode(Object obj, JSONWriter jb) throws IOException {
                jb.valueString(obj.toString());
            }
        };
        GlobalEncoderMap.put(String.class, e);
        GlobalEncoderMap.put(StringBuilder.class, e);
        GlobalEncoderMap.put(StringBuffer.class, e);
        e = new Encoder() {
            @Override
            public void encode(Object obj, JSONWriter jb) throws IOException {
                jb.valueString(Bytes.bytes2base64((byte[]) obj));
            }
        };
        GlobalEncoderMap.put(byte[].class, e);
        e = new Encoder() {
            @Override
            public void encode(Object obj, JSONWriter jb) throws IOException {
                jb.valueString(new SimpleDateFormat(DATE_FORMAT).format((Date) obj));
            }
        };
        GlobalEncoderMap.put(Date.class, e);