check_outputs doesn't find output to subaddress
I'm unable to detect transaction output sent to subaddress using the `TransactionPrefix`'s `check_outputs` method. Full test case: https://gist.github.com/ladislavdubravsky/73a922930e7cbd13b415b908009852d9

I can recognize the output using the library like this:
```let spend_subaddr = get_subaddress_spend_public_key(&view_pair, Index { major: 0, minor: 1});
let spend_subaddr = get_subaddress_spend_public_key(&view_pair, Index { major: 0, minor: 1});
let view_pair_subaddr = ViewPair { view: view, spend: spend_subaddr };
let owned_outputs = tx.prefix.check_outputs(&view_pair_subaddr, 0..10, 0..10);
println!("Owned outputs={:#?}", owned_outputs);  // we get our output!
```

Was check_outputs intended to cover this automatically? If so, what I'm confused by is that it seems like it should happen in the `tx_additional_pubkeys` leg of the function, however, we don't get any additional keys here, as
```
// we don't need to include additional tx keys if:
//   - all the destinations are standard addresses
//   - there's only one destination which is a subaddress
```

(https://github.com/monero-project/monero/blob/master/src/cryptonote_core/cryptonote_tx_utils.cpp#L396). Any feedback is appreciated!