[ietf-dkim] z= question with X headers

[Eric Allman]

> Perhaps:
>
> "A vertical-bar-separated list of select header field names and
> copies of header field values that identify the header fields
> present when the message was signed.  It is not required to include
> all header field names and values."

I've added essentially this wording.  Sorry for the confusion; it was 
definitely ambiguous.

eric