Add disable_popularity debug parameter

Adding debug=disable_popularity disables the use of popularity
information when searching.  This is useful for checking the effect of
popularity information on searches.  This can be combined with the
existing disable_best_bets by joining the options with a comma
separator.
Whitespace fix
Use .to_s instead of string interpolation

This is a bit clearer in intent.
Use refute_match instead of assert !~
Merge pull request #239 from alphagov/add_disable_popularity_debug_option

Add disable_popularity debug parameter