mailinfo: do not concatenate charset= attribute values from mime headers

mailinfo: do not concatenate charset= attribute values from mime headers

"Content-type: text/plain; charset=UTF-8" header should not appear
twice in the input, but it is always better to gracefully deal with
such a case.  The current code concatenates the value to the values
we have seen previously, producing nonsense such as "utf8UTF-8".

Instead of concatenating, forget the previous value and use the last
value we see.

Signed-off-by: Junio C Hamano <gitster@pobox.com>