Resets current user.

--HG--
branch : ../sandbox/rails-4.1
extra : convert_revision : svn%3Ae93f8b46-1217-0410-a6f0-8f06a7374b81/sandbox/rails-4.1%4013447
Fixed that Issue.visible with an unsaved user triggers an SQL error.

--HG--
branch : ../sandbox/rails-4.1
extra : convert_revision : svn%3Ae93f8b46-1217-0410-a6f0-8f06a7374b81/sandbox/rails-4.1%4013448