Check edit tags is a list (#1287)

Libraries have a `collections()` method which has the same name as media's `collections` attribute.

Author: JonnyWong16

plexapi/mixins.py

@@ -792,7 +792,8 @@ def editTags(self, tag, items, locked=True, remove=False, **kwargs):
 
         if not remove:
             tags = getattr(self, self._tagPlural(tag), [])
-            items = tags + items
+            if isinstance(tags, list):
+                items = tags + items
 
         edits = self._tagHelper(self._tagSingular(tag), items, locked, remove)
         edits.update(kwargs)