serialize_tags: make .filter(tags__contains='string') work

the example in readme says this example works, but actually it splits
the argument by ',' unconditionally, expecting a list.  so a string will
be made into a "s,t,r,i,n,g".  so we should handle getting passed either
a single string and pass it through unchanged, or a list and join them

actually if a list, we'd want to dedupe so we use a set which is the
native type of the library for tags anyways

Author: smemsh

tasklib/serializing.py

@@ -175,7 +175,13 @@ def deserialize_annotations(self, data):
         return [TaskAnnotation(self, d) for d in data] if data else []
 
     def serialize_tags(self, tags):
-        return ','.join(tags) if tags else ''
+        if isinstance(tags, list):
+            tags = set(tags)
+        if isinstance(tags, set):
+            return ','.join(tags) if tags else ''
+        if isinstance(tags, str):
+            return tags
+        raise ValueError("serialize_tags only supports list, set or string")
 
     def deserialize_tags(self, tags):
         if isinstance(tags, str):