prevent headers justification

[Sébastien Kirche]

Hi,

while trying to change the format or my gnus-user-agent from (emacs gnus
type) to (emacs  gnus config), I noticed that gnus  justifies some of my
headers.

It  is  probably unrelated  to  specific  User-Agent:  header (with  the
previous gnus-user-agent value, the header contained 

User-Agent: Gnus/5.110004 (No Gnus v0.4) Emacs/22.0.50 (darwin)

and it was not justified. 
As far I can see it justifies also the references: header.

While i am typing my messages, I have the filladapt and auto-fill modes.
How could I prevent my headers from being justified ?

Thanks for some pointers.
-- 
Sébastien Kirche