ForEach mutates empty AMT

[acruikshank]

Version: github.com/filecoin-project/go-amt-ipld/v2 v2.0.1-0.20200424220931-6263827e49f2

Problem:
Flushing an empty AMT produces a different CID before and after a ForEach is run on it. The following test fails:

func TestEmptyAMTRoot(t *testing.T) {
	ctx := context.Background()
	rt := mock.NewBuilder(context.Background(), address.Undef).Build(t)
	store := adt.AsStore(rt)
	root := amt.NewAMT(store)

	cid1, err := root.Flush(ctx)
	require.NoError(t, err)

	root.ForEach(ctx, func(_ uint64, _ *cbg.Deferred) error {
		return nil
	})

	cid2, err := root.Flush(ctx)
	require.NoError(t, err)

	assert.Equal(t, cid1, cid2)
}

The problem appears to be that the expVals slice is empty before the ForEach and is populated with 8 nil values after.