Python 3 fix: FilterConverter sub-classing bug

Couldn't be instantiated more than once. This made it impossible to make a subclass of FilterConverter.

flask_admin/model/filters.py

@@ -266,7 +266,7 @@ def convert(*args):
         See :mod:`flask.ext.admin.contrib.sqla.filters` for usage example.
     """
     def _inner(func):
-        func._converter_for = map(str.lower, args)
+        func._converter_for = list(map(str.lower, args))
         return func
     return _inner