option_groups_from_collection_for_select should produce an HTML-safe string

Can somebody please review my ticket:

  https://rails.lighthouseapp.com/projects/8994/tickets/4879

It's a trivial bugfix ("option_groups_from_collection_for_select
should produce an HTML-safe string") before RC, I think.

Cheers,
Wincent

Applied, thank you!