Fix #36 (experimental): batch writes shouldn't range over sets

Untested fix, written in a hurry.

Batch requests like `{:mytable {:put [{:id 1 :s #{"a" "b"}}` were
incorrectly ranging over individual s values: "a", "b" rather than
being correctly interpreted as a _single_ :put with a single set
value.
v1.5.0-SNAPSHOT