Pass the debug parameter through to rummager

This parameter allows the ranking options to be changed; for example, it
allows use of best-bets to be turned off.  It's useful to be able to
make these modifications on the live search results to compare the final
ranking that would be produced, and to check if options like popularity
information help for particular searches.
Merge pull request #620 from alphagov/pass_debug_to_search

Pass the debug parameter through to rummager