Filtering clause returns unexpected failure

Given the following data set

```
/_ "_predicate"@[] "/some/immutable/id"@[]
/_ "_owner"@[2017-05-23T16:41:12.187373-07:00] /gid<0x9>
/_ "_subject"@[] /aid
/_ "_object"@[] /aid
/_ "_object"@[2017-05-23T16:41:12.187373-07:00] /aid
/_ "_owner"@[2017-05-23T16:41:12.187373-07:00] /gid<0x6>
/_ "_predicate"@[2017-05-23T16:41:12.187373-07:00] "/some/temporal/id"@[2017-05-23T16:41:12.187373-07:00]
/_ "_subject"@[2017-05-23T16:41:12.187373-07:00] /aid
/aid "/some/temporal/id"@[2017-05-23T16:41:12.187373-07:00] /aid
/aid "/some/immutable/id"@[] /aid
/aid "/some/ownerless_temporal/id"@[2017-05-23T16:41:12.187373-07:00] /aid
```

The following query succeeds as expected.

```
bql> SELECT ?bn,?p, ?o
FROM ?test
WHERE {
?bn "_subject"@[,] /aid.
?bn "_predicate"@[,] ?p .
?bn "_object"@[,] ?o
};

?bn ?p ?o
/_ "/some/temporal/id"@[2017-05-23T16:41:12.187373-07:00] /aid

[OK] Time spent: 578.963µs
```

However, when you specify ?o, it fails with a filtering error.

```
bql> SELECT ?bn,?p
FROM ?test
WHERE {
?bn "_subject"@[,] /aid.
?bn "_predicate"@[,] ?p .
?bn "_object"@[,] /aid
};

[ERROR] planner.Execute: failed to execute insert plan with error failed to fully specify clause { ?bn "_object"@[,] /aid } for row map[?bn:/_]
Time spent: 514.294µs
```

Given the this is just and update on the query above, this should have not failed and return one row with `?bn` and `?p` bindings.