[ietf-dkim] Issue 1543: remove [FWS]

[Frank Ellermann]

Jim Fenton wrote:
 
> Thoughts?

Close the ticket as soon as *FWS is replaced by either
*WSP or [FWS].  Maybe add text in the direction of...

| [FWS] is expected to be replaced by *SP or similar
| in a future update of [RFC4871].

...for the [FWS] variant.  The TAB in *WSP is also not
more state of the art (as documented in RFC 5198), so
maybe 4871bis will pick *SP after/if FWS failed in the
implementation and interop tests for a draft standard.

 Frank