(spam-generate-fake-headers): rewrite to be simpler,

(spam-generate-fake-headers): rewrite to be simpler,
also under 80 char limit, and call gnus-error if needed
(spam-fetch-article-header): finally fixed - it was a
buffer-local variable (gnus-newsgroup-data)
(spam-find-spam): use spam-generate-fake-headers, forget about
spam-insert-fake-headers
(spam-insert-fake-headers): removed