db.batch().put() does not honor original options (like valueEncoding)

I encountered this when using `valueEncoding: "utf8"` on the levelup constructor, and then running `db.batch().put().write()` -- my values were just the Object.toString().  All this does is make the batch API use the same options call that the main levelup path does.
Merge pull request #215 from mcavage/patch-1

db.batch().put() does not honor original options (like valueEncoding)