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.
assert_no_match isn't available here:

    test: search with debug disabling use of popularity should have not have a custom_score clause to add popularity in payload. (UnifiedSearcherBuilderTest):
    NoMethodError: undefined method `assert_no_match' for #<UnifiedSearcherBuilderTest:0x00000004f990a8>

I've changed it to use .to_s, though.