issue #69 添加Session的支持，添加测试用例

weixin-java-common/src/main/java/me/chanjar/weixin/common/util/WxMessageInMemoryDuplicateChecker.java

@@ -82,6 +82,9 @@ public class WxMessageInMemoryDuplicateChecker implements WxMessageDuplicateChec
 
   @Override
   public boolean isDuplicate(Long wxMsgId) {
+    if (wxMsgId == null) {
+      return false;
+    }
     checkBackgroundProcessStarted();
     Long timestamp = msgId2Timestamp.putIfAbsent(wxMsgId, System.currentTimeMillis());
     if (timestamp == null) {