fix(3p/lisp/mime4cl): don't store redundant headers in MIME-MESSAGE r/8975

MIME-MESSAGE has a HEADERS slot which is an alist of all headers. Some
of those headers will be parsed again and stored in MIME-PART (or a
subclass of it). Having the header content stored in the HEADERS alist
and in MIME-PART causes problems:

- Requires extra knowledge about how messages are parsed when rendering
  messages.
- Makes MIME= depend on the specific whitespace and quoting in those
  headers which isn't preserved by how mime4cl parses e.g. Content-Type.
- Gives users two ways that slightly diverge to access the same thing.

To avoid this, we remove these headers after the MIME-PARTs contained in
MIME-MESSAGE have been initialized (since they reuse the HEADERS slot).

Change-Id: I5b221f88bbac47dd81db369e3c1d5881a5a50e5e
Reviewed-on: https://cl.tvl.fyi/c/depot/+/12858
Tested-by: BuildkiteCI
Reviewed-by: sterni <sternenseemann@systemli.org>
Autosubmit: sterni <sternenseemann@systemli.org>

0 files changed, 0 insertions, 0 deletions