boost: use utf-8 encoding for unknown locales.

boost: use utf-8 encoding for unknown locales.

More exotic locales like es-419 cannot be parsed by boost, so we
fall-back to the default encoding. This avoids an exception:
invalid_charset_error of the form:

"Invalid or unsupported charset:us-ascii or UTF-8"

for this case.

Change-Id: I6796dd893ec774b221956ea9febbcc19495d47b5
Reviewed-on: https://gerrit.libreoffice.org/c/core/+/130102
Tested-by: Jenkins
Reviewed-by: Michael Meeks <michael.meeks@collabora.com>
Reviewed-on: https://gerrit.libreoffice.org/c/core/+/130146
Reviewed-by: Andras Timar <andras.timar@collabora.com>