Fix automatic block size detection for leaf spaces

The logic in ISTL::VectorBackend was not adjusted correctly when introducing the automatic block size detection, causing a problem when using Blocking::fixed and automatic block size detection on leaf spaces: the ordering infrastructure would work in non-blocking mode, while the created container had block structure.

Fixed by incorporating the new special case of block_size == 0, which signifies automatic block size detection.

This MR also adds a test to catch similar problems in the future.

Closes #114 (closed).