NoMethodError

[mmackiew]

1. Download plugin in zip format.
2. Unpack to REDMINE/plugins/spectator
3. Click on user login to be his spectator generate error in production.log file:

Parameters: {"id"=>"79"}
Current user: po_mmackiew (id=6)
Completed 500 Internal Server Error in 4ms (ActiveRecord: 0.8ms)

NoMethodError (undefined method generate_session_token' for #<User:0x007f4c1569f088>): lib/redmine/sudo_mode.rb:63:in sudo_mode'

Installed Redmine:
Environment:
Redmine version 3.1.2.stable
Ruby version 2.1.2-p95 (2014-05-08) [x86_64-linux]
Rails version 4.2.4
Environment production
Database adapter PostgreSQL
SCM:
Subversion 1.8.8
Mercurial 2.8.2
Git 1.9.1
Filesystem