improve implemetation fix broken UT: avoid child thread update parent thread TTL holder

Author: oldratlee

src/main/java/com/alibaba/ttl/TransmittableThreadLocal.java

@@ -97,6 +97,11 @@ T copyValue() {
                 protected Map<TransmittableThreadLocal<?>, ?> initialValue() {
                     return new WeakHashMap<TransmittableThreadLocal<?>, Object>();
                 }
+
+                @Override
+                protected Map<TransmittableThreadLocal<?>, ?> childValue(Map<TransmittableThreadLocal<?>, ?> parentValue) {
+                    return new WeakHashMap<TransmittableThreadLocal<?>, Object>(parentValue);
+                }
             };
 
     void addValue() {