Sweden does not offer habeus corpus to detainees who are under investigation but have not been charged. This is the category in which they placed Assange.

https://wikileaks.org/Sweden-Tells-the-UN-that.html

The Swedish desire to maneuver Assange into a black hole is self-evident in its refusal to interview Assange by telephone. Once that was done they would have had to charge him and present their evidence to the public under Swedish law.