fix some issues

demo/test.yaml

@@ -14,14 +14,16 @@ fields:
     use: small,large
     loop: 3
     loopfix: " "
-    postfix: "            "
+    prefix: "["
+    postfix: "]"
     expect: 1,2,3,101,102...
 
   - field: field2
     note: 引用內置IP地址数据定义中的C类地址
     from: system.ip.v1.yaml
     use: privateC
-    prefix: _
+    prefix: "["
+    postfix: "]"
     loop: 3
     loopfix: "  "
     expect: 192.168.0.1 ...​