KeyError when query includes "UNION"

pretty much the title.

I'm a big fan of this package, it helped me alot in my Masterthesis but today I seemed to have trouble with it.

```
from aiosparql.client import SPARQLClient

async with SPARQLClient("https://dbpedia.org/sparql") as client:
result = await client.query("select ?redirects where {{?redirects dbo:wikiPageRedirects dbr:Big_Trout_Lake} union {dbr:Big_Trout_Lake dbo:wikiPageRedirects ?redirects}}")
```

this query seems to break and infact every query that includes "UNION". For now I will try to work around it by quering the endpoint two times and concatinate the results afterwards but it would be cool if you could fix that issue.