Update scoreable to return a large positive number when score==0 (#290)

Avoid infinite activity score orders

Items with a zero activity score were getting an activity sorting score
of -infinity, because we base the value on log2(score). Adjusting the
calculation avoids this edge case, by always basing the sorting score on
a non-zero number.

---------

Co-authored-by: Kevin McConnell <kevin@37signals.com>

app/models/bubble/scorable.rb

@@ -45,7 +45,7 @@ module Bubble::Scorable
       # We store this as log2 to tame the huge/tiny numbers we'd otherwise get
       # when activity is far from the reference date.
       days_diff = (activity_at - REFERENCE_DATE) / 1.day
-      Math.log2(score) + days_diff
+      Math.log2(1.0 + score) + days_diff
     end
 
     def last_scorable_activity_at

test/models/bubble/scorable_test.rb

@@ -53,4 +53,13 @@ class Bubble::ScorableTest < ActiveSupport::TestCase
       assert_equal [ bubble_new, bubble_mid, bubble_old ], Bubble.where(id: [ bubble_old, bubble_mid, bubble_new ]).ordered_by_activity
     end
   end
+
+  test "bubbles with no activity have a valid activity_score_order" do
+    bubble = Bubble.create! bucket: buckets(:writebook), creator: users(:kevin)
+
+    bubble.rescore
+
+    assert bubble.activity_score.zero?
+    assert_not bubble.activity_score_order.infinite?
+  end
 end