BUGFIX: tie to CharField if separator given

Author: Javier Palacios

src/django_mysql/models/aggregates.py

@@ -25,7 +25,9 @@ def __init__(
     ):
 
         if "output_field" not in extra:
-            if distinct:
+            if separator:
+                extra["output_field"] = CharField()
+            elif distinct:
                 extra['output_field'] = SetCharField(CharField())
             else:
                 extra['output_field'] = ListCharField(CharField())