Fixes https://github.com/MetaModels/filter_register/issues/10

@MarkejN fix author validation https://travis-ci.com/github/MetaModels/filter_register/jobs/501127959#L870-L871

@discordier what do you think - in MM 2.2 it works without problems with umlauts

The change is correct non-the-less, as we should always correctly operate on multibyte chars

fix the authoring problems and this is good to go.