v0.7.1 wip

Fixed two bugs in the SortJoinStream class: the first was highlighted in levelgraph-jsonld and it created the
'homes in paris' test case. It was caused by a too early close of the internal stream.
The second was caused by a wrong skipping of triples in the merge sort implementation.

The first is a regression introduced in v0.7.0, while the second was already there.

@elf-pavlik can you please test this and confirm everything works as planned?