tedious
Encoding lower/uppercase
Some mail clients report the encoding in lowercase ('koi8-r' for Yandex.ru, for instance). This causes an error in Message::processStructure since it appears in uppercase in mb_list_encodings(). The resulting fallback to UTF-8 makes the final message unreadable.
I suggest testing for both lower and uppercase versions of the encoding's name, because mb_convert_encoding seems perfectly fine with both versions.
@dratel29 can you test with https://github.com/linniksa/Fetch/commits/patch-generalize-charset-convert ?
+1 on this. Just run into the same issue, where a client sends iso-8859-1 as charset. I like the solution from @linniksa.
