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

README.rst

@@ -36,7 +36,7 @@ tasklib has a similar API to that of Django's ORM::
     >>> tasks = tw.tasks.pending()
     >>> tasks
     ['Tidy the house', 'Learn German']
-    >>> tasks.filter(tags__contain='chores')
+    >>> tasks.filter(tags__contains='chores')
     ['Tidy the house']
     >>> type(tasks[0])
     <class 'tasklib.task.Task'>

tasklib/serializing.py

@@ -174,7 +174,13 @@ 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):