Fixed unicode decode bug in task overview.

Fixes https://github.com/4teamwork/opengever.phvs/issues/27

Bug happens only in single client setups. He appears if a task is displayed who has a special character in its title or its breadcrumb.

@lukasgraf 
If you absolutely *have* to do type checking, use [`isinstance()`](https://docs.python.org/2/library/functions.html#isinstance), not `type(x) == y`.