jasl#14 add migration file. improve method set_time_zone.

app/controllers/application_controller.rb

@@ -60,6 +60,6 @@ def #{s}!(redirect_url: nil)
 
   private
     def set_time_zone
-      Time.zone = current_user.time_zone if current_user and current_user.time_zone.present?
+      Time.zone = current_user.time_zone if current_user&.time_zone&.present?
     end
 end

db/migrate/20190809022738_add_user_time_zone.rb

@@ -0,0 +1,5 @@
+class AddUserTimeZone < ActiveRecord::Migration[6.0]
+  def change
+    add_column :users, :time_zone, :string
+  end
+end

db/migrate/20190814094307_add_user_time_zone.rb

@@ -0,0 +1,5 @@
+class AddUserTimeZone < ActiveRecord::Migration[6.0]
+  def change
+    add_column :users, :time_zone, :string
+  end
+end