bug#33313: unwrapped headers (both after posting-styles insertion and before send)

[Garreau, Alexandre]

Using the ".*"-keyed `gnus-posting-styles' element, I have set my
User-Agent header to (format "Gnus (%s), %s" gnus-version-number
(emacs-version)), so it gets a really long line and a newline in it.  I
would have expected it to be wrapped after inserting in a new message
buffer.  Otherwise I may have expected for headers to be wrapped before
to be sent.

Anyway I find dirty and redundant to require users to know headers
correct wrapping rules so to set them (since there’s nothing more
high-level for it).

Gnus v5.13
GNU Emacs 25.1.1 (i686-pc-linux-gnu, GTK+ Version 3.22.11)
 of 2017-09-15, modified by Debian