Fix contributions calendar empty problem under mysql

When using MySQL as database backend in GitLab, ``date`` in ``date(created_at), count(id) as total_amount``
won't return the ``date`` column (should be ``date(created_at)``), as a result, there's no contribution in the user
profile page.
Adding an ``as date`` can solve this problem.

lib/gitlab/contributions_calendar.rb

@@ -17,7 +17,7 @@ module Gitlab
       events = Event.reorder(nil).contributions.where(author_id: user.id).
         where("created_at > ?", date_from).where(project_id: projects).
         group('date(created_at)').
-        select('date(created_at), count(id) as total_amount').
+        select('date(created_at) as date, count(id) as total_amount').
         map(&:attributes)
 
       dates = (1.year.ago.to_date..(Date.today + 1.day)).to_a