Marking `super()` calls `safe` by default

I've found that with `autoescape` turned on, all `{{ super() }}` calls have to be made explicitly `safe`. Is this deliberate, or an oversight? I'd have thought that it would make more sense to assume anything coming back from `super()` would be fine, but interested to know your ideas.

Returning a `runtime.SafeString` from [`getSuper`](https://github.com/jlongster/nunjucks/blob/master/src/environment.js#L224-L236) seems to resolve this, but it may not be the best place, or best way, to resolve this issue, if it is an issue at all.