Add tag to tag serialization

This omission was causing "tag" change objects to be deserialized
from the change cache incorrectly.

Change-Id: Idb5f1b3cd4e9eead01bd8354427e7e78bb2e6c8b

zuul/model.py

@@ -5176,11 +5176,13 @@ class Tag(Ref):
     def serialize(self):
         d = super().serialize()
         d["containing_branches"] = self.containing_branches
+        d["tag"] = self.tag
         return d
 
     def deserialize(self, data):
         super().deserialize(data)
         self.containing_branches = data.get("containing_branches")
+        self.tag = data.get("tag")
 
 
 class Change(Branch):