I _think_ that this should be a behavior specified in AbstractLevelDown in the `batch` method.
This should be consistent across all write APIs, if we specify a default for the `'type'`, it should be everywhere.
It's more DRY.

BTW, I'm more in favor of emitting errors: if an user is passing something else other than `'del'` or `'put'`, he is clearly doing something wrong.