Properly marshal extensions list

Author: jtc42

src/labthings/default_views/extensions.py

@@ -17,4 +17,4 @@ def get(self):
         Returns a list of Extension representations, including basic documentation.
         Describes server methods, web views, and other relevant Lab Things metadata.
         """
-        return registered_extensions().values() or []
+        return self.schema.dump(registered_extensions().values() or [])