http: do not assign string constant to non-const field

http: do not assign string constant to non-const field

In `write_accept_language()`, we put all acceptable languages into an
array. While all entries in that array are allocated strings, the final
entry in that array is a string constant. This is fine because we
explicitly skip over the last entry when freeing the array, but will
cause warnings once we enable `-Wwrite-strings`.

Adapt the code to also allocate the final entry.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>