Returns tags through WebDAV

Fixes https://github.com/owncloud/core/issues/12856

I figured out how to make it return a list of tags :smile: 

@DeepDiver1975 @dragotin @icewind1991 

- [x] only select/return tags when explicitly requested
- [x] separate tags from favorite ?
- [x] unit tests
Very strange, but this works. Maybe not strict PHP.
I'll replace it.