Make Date header not required

[Sergey Ponomarev <[e-mail]>]

The Date header MUST be added to the response but at the same time it may
be omitted if the origin server doesn't have a good clock. It looks like
the header is useful only if server or client have misconfigured time but
the server also returns an Expires header that uses a fixed date.
That's why the header was replaced with more robust Cache-Control that uses
max-age with relevant time instead of fixed.

Also another real life example of usage of the Date is some clients that
have blocked NTP and they tried to fetch time from google.com or 1.1.1.1.

Both examples of usage are not so popular and in most cases the Date header
is just not needed.
But when implementing a web server you want to be compliant with spec or
just don't know how and where the Date is used exactly.
In the next release of BusyBox httpd server for embedded devices the Date
header can be disabled during a compilation. But this will make the server
non RFC compatible.

Is it possible to change MUST to MAY for the header? And add a note when
the header is really needed.

Regards,
Sergey Ponomarev