Serialising a double inverse property path results in a syntactically invalid query

When serialising the following query, the surrounding `(` `)` are forgotten, causing a syntactically invalid query to be generated:

```rq
ASK WHERE { ?s ^(^<a:a>) ?o. }
```

becomes:

```rq
ASK WHERE { ?s ^^<a:a> ?o. }
```

which is syntactically invalid.

#165 contains a breaking test for this case.