UnicodeEncodeError on export

While trying to export text fields, I get:

`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 66, in __call__
Module plone.z3cform.layout, line 50, in update
Module plone.z3cform.fieldsets.extensible, line 59, 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 21, 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.importexport.browser.import_view, line 675, in handleExport
Module collective.importexport.browser.import_view, line 267, in export_file`
```

`UnicodeEncodeError: 'ascii' codec can't encode character u'\xd6' in position 3: ordinal not in range(128)`

This can be solved by inserting a new line 264 in collective.importexport.browser.import_view:

`value = value and value.encode('utf-8')`

Start in collective.importexport.browser.import_view at export_file around line 267, following the traceback from handleExport. Reproduce the export with text containing Ö and verify that the resulting CSV handles the character without UnicodeEncodeError. Done means the export succeeds with non-ASCII text.

Written by the indexing model from the issue text.