When you have searched for a keyword, browsed to another batch in that result set starting at b_start=n, then select a facet with < n results, you end up with an exception, because you send a query to SOLR for a result set start at n and SOLR only returns empty results at that moment.
The easiest solution is to clear b_start when searching for facets.
Remove a b_start parameter when generating facet urls.
This has the intended effect of resetting the batch to the first page, so we do not error out when a facet leads to a search result set smaller than the current batch page total. Fixes #12.