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)
```
d = new Decoder() {
            @Override
            public Object decode(Object jv) throws IOException {
                return new StringBuffer(jv.toString());
            }
        };
        GlobalDecoderMap.put(StringBuffer.class, d);
        d = new Decoder() {
            @Override
            public Object decode(Object jv) throws IOException {
                if (jv instanceof Number) return BigInteger.valueOf(((Number) jv).longValue());
                return (BigInteger) null;
            }
        };
        GlobalDecoderMap.put(BigInteger.class, d);
        d = new Decoder() {
            @Override
            public Object decode(Object jv) throws IOException {
                if (jv instanceof Number) return BigDecimal.valueOf(((Number) jv).doubleValue());
                return (BigDecimal) null;
            }
        };
        GlobalDecoderMap.put(BigDecimal.class, d);
        d = new Decoder() {
            @Override
            public Object decode(Object jv) throws IOException {
                if (jv instanceof Number) return new AtomicInteger(((Number) jv).intValue());
                return (AtomicInteger) null;
            }
        };