Close after slurping (fixes SI-7244)

Since slurp by definition is the noise made by the straw when
the cup is empty and all milk shake is consumed, we can safely
close after slurping.

Use case was deleting a test artifact on Windows.

Speaking of testing, it's not obvious that this change wants a partest test.  But if instructed where to put it, I would add a unit test, perhaps someplace friendly to sbt.