Add support for composite columns

Author: atugushev

sqlalchemy_utils/listeners.py

@@ -9,6 +9,9 @@ def coercion_listener(mapper, class_):
     capable type.
     """
     for prop in mapper.iterate_properties:
+        if not prop.columns:
+            continue
+
         try:
             listener = prop.columns[0].type.coercion_listener
         except AttributeError: