bug#31651: 26.1.50; message-forward-included-headers should contain "^$"

[Katsumi Yamaoka]

Hi,

The docstring of `message-forward-included-headers' says:

  This variable should be a regexp or a list of regexps.

But if it is a regexp, it should contain "^$" like:

"^$\\|^\\(?:Cc\\|Date\\|From\\|In-Reply-To\\|Message-Id\\|References\\|Reply-To\\|Subject\\|To\\|User-Agent\\|X-Mailer\\):"

Or the header separator in a forward message will be removed
and the message will get malformed.

A better way to solve this issue would be to improve the
`message-remove-ignored-headers' code rather than adding a causion
to the docstring of `message-forward-included-headers', I think.

I use `message-forward-as-mime' with t as it was before.

Thanks.