back_to_search should return the catalog path if last search is nil
presently it just links to the current document url.
This doesn't present itself in blacklight, because the back_to_search is
guarded in the view and doesn't display unless there is a next or
previous document. In my custom app, I want to put it on the page
regardless.
Merge pull request #951 from projectblacklight/back_to_nil

back_to_search should return the catalog path if last search is nil