add tests, remove storeObjects, fix case with both subfolder and document
Added some more unit tests (still a couple to go).
I removed the storeObjects method because it makes no sense to have it
at the PrefixTree level; if we want to have it, then it should go on
the baseClient level (and PrefixTree should just pass it through like
for other methods).
One test I added actually failed, because it prefers to go deeper if
given the option. Instead, it should return f/o/_o if it exists,
even if f/o/o/ also exists.