Bugfix: ComposedOperator now uses the default_spaces attribute.

nifty/operators/composed_operator/composed_operator.py

@@ -96,6 +96,8 @@ class ComposedOperator(LinearOperator):
         is not easily forecasteable what the output of an operator-call
         will look like.
         """
+        if spaces is None:
+            spaces = self.default_spaces
         return spaces
 
     # ---Mandatory properties and methods---