Mail Subject for non Text field

[tomgross]

Using a non-text / non-string field (eg a multi choice field) as "Use-as-Subject"-field for the mailing action throws an error:

Traceback (innermost last):
  Module ZPublisher.Publish, line 138, in publish
  Module ZPublisher.mapply, line 77, in mapply
  Module ZPublisher.Publish, line 48, in call_object
  Module plone.z3cform.layout, line 63, in __call__
  Module plone.z3cform.layout, line 47, in update
  Module collective.easyform.browser.view, line 266, in update
  Module plone.z3cform.fieldsets.extensible, line 65, in update
  Module plone.z3cform.patch, line 30, in GroupForm_update
  Module z3c.form.group, line 145, in update
  Module plone.app.z3cform.csrf, line 22, in execute
  Module z3c.form.action, line 98, in execute
  Module z3c.form.button, line 315, in __call__
  Module z3c.form.button, line 170, in __call__
  Module collective.easyform.browser.view, line 158, in handleSubmit
  Module collective.easyform.browser.view, line 128, in processActions
  Module collective.easyform.actions, line 510, in onSuccess
  Module collective.easyform.actions, line 446, in get_mail_text
  Module collective.easyform.actions, line 392, in get_header_info
TypeError: coercing to Unicode: need string or buffer, set found