fix options in ToString

Author: mmatera

mathics/builtin/strings.py

@@ -1604,7 +1604,7 @@ def apply(self, value, evaluation, **options):
         'ToString[value_, OptionsPattern[ToString]]'
         encoding = options['options']["System`CharacterEncoding"]
         text = value.format(evaluation, 'System`OutputForm').boxes_to_text(
-            evaluation=evaluation, encoding=encoding)
+            evaluation=evaluation, encoding=encoding.value)
         return String(text)