fix #13531 by adding a test (#13581)

tests/stdlib/tjson_unmarshall.nim

@@ -0,0 +1,31 @@
+discard """
+  output: '''
+Original: (kind: P, pChildren: @[(kind: Text, textStr: "mychild"), (kind: Br)])
+jsonNode: {"kind":"P","pChildren":[{"kind":"Text","textStr":"mychild"},{"kind":"Br"}]}
+Reversed: (kind: P, pChildren: @[(kind: Text, textStr: "mychild"), (kind: Br)])
+'''
+"""
+
+import json
+
+type
+  ContentNodeKind* = enum
+    P,
+    Br,
+    Text,
+  ContentNode* = object
+    case kind*: ContentNodeKind
+    of P: pChildren*: seq[ContentNode]
+    of Br: nil
+    of Text: textStr*: string
+
+let mynode = ContentNode(kind: P, pChildren: @[
+  ContentNode(kind: Text, textStr: "mychild"),
+  ContentNode(kind: Br)
+])
+ 
+echo "Original: " & $mynode
+
+let jsonNode = %*mynode
+echo "jsonNode: " & $jsonNode
+echo "Reversed: " & $jsonNode.to(ContentNode)