Add test code for a missing branch of Tag.wiki_markup.setter; cleanup.

mwparserfromhell/nodes/tag.py

@@ -223,7 +223,7 @@ class Tag(Node):
     def wiki_markup(self, value):
         self._wiki_markup = str(value) if value else None
         if not value or not self.closing_wiki_markup:
-            self.closing_wiki_markup = str(value) if value else None
+            self._closing_wiki_markup = self._wiki_markup
 
     @self_closing.setter
     def self_closing(self, value):

tests/test_tag.py

@@ -246,6 +246,9 @@ class TestTag(TreeEqualityTestCase):
         node.closing_wiki_markup = "|}"
         self.assertEqual("|}", node.closing_wiki_markup)
         self.assertEqual("{|\n|}", node)
+        node.wiki_markup = "!!"
+        self.assertEqual("|}", node.closing_wiki_markup)
+        self.assertEqual("!!\n|}", node)
         node.wiki_markup = False
         self.assertFalse(node.closing_wiki_markup)
         self.assertEqual("<table>\n</table>", node)