Use squish to also compact the string

Feedback:
https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/9219#note_23436980

app/models/application_setting.rb

@@ -271,7 +271,7 @@ class ApplicationSetting < ActiveRecord::Base
 
   def default_artifacts_expire_in=(value)
     if value.present?
-      super(value.strip)
+      super(value.squish)
     else
       super(nil)
     end