switch to unquote_plus in bucketlistresultset

There is a bug in PR #2562, which I submitted. The URL quoting that S3 performs with `encoding_type="url"` necessitates the use of `unquote_plus` because they replace spaces with plus signs. This didn't break bucket listing completely, and in my case it only led to some keys missing from the listing, something that I didn't notice in my original testing. Sorry for this!