Fix DirectMessage.today on different time zones

The dates are saved on UTC times on the database. So, for example,
if living in West Australia, `Date.current.beginning_of_day` will be
stored as UTC's yesterday at 15:15:00, while `Date.current.end_of_day`
will be stored as UTC's today at 15:14:59.

When we use the `DATE` database function, PostgreSQL will select the
records with the same UTC date as the current UTC date. However, we need
the records with the same application date (as defined in
`config.time_zone`) as the current application date. The test passed
(for us) because we were using `beginning_of_day + 3.hours` to make sure
we were creating records when the date in Madrid was the same as the UTC
date.

Using a ruby interval for the time condition solves the problem.

app/models/direct_message.rb

@@ -8,7 +8,7 @@ class DirectMessage < ApplicationRecord
   validates :receiver, presence: true
   validate  :max_per_day
 
-  scope :today, lambda { where("DATE(created_at) = DATE(?)", Time.current) }
+  scope :today, lambda { where(created_at: Date.current.beginning_of_day..Date.current.end_of_day) }
 
   def max_per_day
     return if errors.any?