Make IExactIndexer call `self.unescape` in `pre_value_for_query`.
Make IExactIndexer call `self.unescape` in `pre_value_for_query`.

This fixes a bug where it didn't work if there were underscores in the value.

grzes/djangae,potatolondon/djangae,kirberich/djangae,kirberich/djangae,grzes/djangae,potatolondon/djangae,grzes/djangae,kirberich/djangae