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)
```
public class JSONTest {
    static byte[] DEFAULT_BYTES = {3, 12, 14, 41, 12, 2, 3, 12, 4, 67, 23};
    static int DEFAULT_$$ = 152;
    @Test
    public void testException() throws Exception {
        MyException e = new MyException("001", "AAAAAAAA");
        StringWriter writer = new StringWriter();
        JSON.json(e, writer);
        String json = writer.getBuffer().toString();
        System.out.println(json);
        
        StringReader reader = new StringReader(json);
        MyException result = JSON.parse(reader, MyException.class);
        Assert.assertEquals("001", result.getCode());
        Assert.assertEquals("AAAAAAAA", result.getMessage());
    }
    @Test
    @SuppressWarnings("unchecked")
    public void testMap() throws Exception {
        Map<String, String> map = new HashMap<String, String>();
        map.put("aaa", "bbb");
        StringWriter writer = new StringWriter();
        JSON.json(map, writer);
        String json = writer.getBuffer().toString();
        Assert.assertEquals("{\"aaa\":\"bbb\"}", json);
        StringReader reader = new StringReader(json);