Fix Unicode handling in OGDSUpdater

This PR mainly fixes issues with Unicode handling in `OGDSUpdater`.

In order to properly fix the issue, I created some basic tests for the `OGDSUpdater` and added some additional tests that exercise group and user import with entities that contain non-ASCII characters.

I also sneaked in two additional small fixes:
- Set proper `Content-Type` header in responses from LDAP control panel views
- Fixed argument order in log message during OGDS sync

@phgross @deiferni