Fix assignUserGroup permission serialization bug

[Zae]

The assignUserGroup permission was being serialized as an empty string, now it will serialize as 'assignUserGroup:' and correctly import.

Fixes #150

• [x] I updated the changelog
• [ ] I updated the composer.json version
• [ ] I wrote tests
• [x] I am proud of my code

I could not get the testsuite to work on my local environment so sadly I did not add any tests.