Fix StringIO issue2550713: - io.StringIO in newer versions of python returns unicode strings and expects a unicode string in the constructor. Unfortunately csv doesn't handle unicode (yet). So we need to use a BytesIO which gets the utf-8 string from the web-interface. Compatibility for old versions by using Stringio.Stringio for emulating a io.BytesIO also works. - We didn't have a regression test for the EditCSVAction

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