Add a Dataloader.KV test to assert that the loader can
load new data in subsequent rounds.

See issue absinthe-graphql#74

Handle case where Dataloader.KV has some results and is asked
to load an id that is neither present in 'results' or 'batches'.

In this case Map.get/2 would return `nil`, which was falling
through and being treated as if the id was found in the results.

This replaces Map.get/2 with Map.get/3 so that we can provide
an explicit value to pattern-match on (:error).

Calling Map.get/3 with :error as the third argument is
almost the same as calling Map.fetch/2 except that non-errors
are wrapped in :ok tuples.

This more clearly delinates the cases where a result is found
and the case where a result is not found.  It also shows that
{:error, reason} is expected to be mapped to 'id' in some
situations.