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.
Don't need to interpolate into a string here, and you can use [assert_match](http://apidock.com/ruby/Test/Unit/Assertions/assert_match).