parser: Ignore CFWS in Message-ID header

parser: Ignore CFWS in Message-ID header

We recently started stripping comments and folding white space from the
In-Reply-To and References headers. Do so also for the Message-ID
header. Because of the importance of the Message-ID header, we accept
even non-compliant headers and because we now have a pattern for this,
we also start (re-)accepting non-compliant In-Reply-To headers.

Signed-off-by: Stephen Finucane <stephen@that.guru>
Related: #399