fixed conversion function and made actual/expected consistent

@wasade, this addresses the issue you were having *IF* you suppress the quality score boundary check in ``skbio.parse.sequences.parse_fastq``. I don't think we want to do that, so I think the other issue is that I used quality scores that were out of range here (though they still worked for testing the filter). So, we should probably update these to use offset 33, within range. Can you make those changes since you're currently more familiar with the quality score thresholds than I am?