FIX: ensures timeline_lookup includes last tuple (#11829)

A simplified version of the logic used in the function before my fix is as follow:

```ruby
result = []
things = [0,1,2,3]
max_values = 2
every = (things.size.to_f / max_values).ceil

things.each_with_index do |t, index|
  next unless (t % every) === 0
  result << t
end

p result # [0, 2]
# 3 doesn’t get included
```

The problem is that if you get unlucky two times you won't get last tuple(s) and might get a very erroneous date.

Double unlucky:
- last tuple index % computed every !== 0 and you don't get the last tuple
- the last tuple is related to a post with a very different date than the previous tuples (on year difference in our case)

lib/timeline_lookup.rb

@@ -12,7 +12,10 @@ module TimelineLookup
     last_days_ago = -1
     tuples.each_with_index do |t, idx|
       return result unless t.is_a?(Array)
-      next unless (idx % every) === 0
+
+      if idx != tuples.size - 1
+        next unless (idx % every) === 0
+      end
 
       days_ago = t[1]
 

spec/lib/timeline_lookup_spec.rb

@@ -0,0 +1,17 @@
+# frozen_string_literal: true
+
+require 'rails_helper'
+
+describe TimelineLookup do
+
+  context '.build' do
+    it 'keeps the last tuple in the lookup' do
+      tuples = [
+        [7173, 400], [7174, 390], [7175, 380], [7176, 370], [7177, 1]
+      ]
+
+      expect(TimelineLookup.build(tuples, 2)).to eq([[1, 400], [4, 370], [5, 1]])
+    end
+  end
+
+end