[Users] Simplify Subject Regexp and "Thread using subject..."

[Jason Miller]

According to the FAQ[1] the simplify subject regex will affect the way
messages are threaded when using "Thread using subject in addition to
standard headers"

However, in my testing it has no effect whatsoever; I use claws at work
where about half the employees are on Exchange which kindly refuses to
set any standard in-reply-to headers, so I have lots of e-mail to test
this on, and simplifying the subject does not cause the e-mail to be
threaded by the modified subject.

Is this a regression, was the FAQ always wrong, or am I doing something
wrong?

-Jason

[1] http://www.claws-mail.org/faq/index.php/Interface#How_can_I_remove_mailing_list_information_from_Subject_headers.3F