This patch makes nnmaildir automatically regenerate NOV data when
nnmail-extra-headers has changed.  (It already automatically
regenerates when the message has been edited, so I think that covers
all regeneration needs.)
	* nnmaildir.el (nnmaildir--update-nov): automatically parse
	NOV data out of the message again if nnmail-extra-headers has
	changed.


paul