nnrss-request-article produces multipart articles

[Klaus Straubinger <KSNetz@Arcor.DE>]

Due to some last-minute changes before the release of Gnus 5.10.1, the
function nnrss-request-article now produces multipart/alternative
articles. The reason given for this change was to avoid some
unspecified problems with w3m. In my view, this is no sufficient reason
for such a change. nnrss should always produce text/html articles since
RSS sites use XML/HTML as output format.

Furthermore, the created HTML code in the text/html part is illegal HTML:
text is added after the closing "</html>" tag.

The nnrss-content-function now has a difficult task to do something
sensible like adding the content of the included link.

Some stylistic issues also come to mind:
  - some code lines are longer than 80 characters,
  - the use of "when" instead of "if" in combination with "progn" seems
    preferable,
  - instead of producing multipart headers manually, an analogon to
    the function mm-insert-multipart-headers should be created and used.

-- 
Klaus Straubinger