views/patch: Set correct encoding for patches

views/patch: Set correct encoding for patches

The patch_mbox view returns text/plain data without specifying a character
set, which means clients will assume the default of iso-8559-1 as defined
in the HTTP/1.1 standard. Since the data being returned is in fact utf-8
encoded, set the encoding accordingly in the HTTP Content-Type header.

Reported-by: Jakub Kicinski <kuba@kernel.org>
Suggested-by: Konstantin Ryabitsev <konstantin@linuxfoundation.org>
Signed-off-by: Toke Høiland-Jørgensen <toke@toke.dk>