filter tests: adapt to kmime enforcing explicit Content-Type
KMime since 5.15.2 turns a not-set content type to explicitly set MIME type text/plain with charset us-ascii. See kmime@6d637f9a.
So tests passing data through KMime have to adapt the expected output to the header having that data set.
mailcommon-filter-filteractionencrypttest & mailcommon-filter-filteractiondecrypttest also need that very adaption, but I am not (yet) firm with the encryption stuff, so better be done by someone who is experienced. (also happy to try myself if given directions how to do it).