Tests for target model

created_at and due_date are preset cause we don't want it to depend
on system time.

app/models/target.rb

@@ -64,7 +64,7 @@ class Target < ActiveRecord::Base
       'unit' => unit,
       'created_at' => created_at,
       'progress' => progress,
-      'days_left' => days_left,
+      'days_left' => days_left
     }
     result
   end