The MessagePack spec allows ill-formed UTF-8 in a str, so the default stays
keep (also if JSON_STRICT_BINARY_UTF8 is enabled), but decoders such as
msgpack-python reject it by default. With the parameter, strict catches
such strings and replace/ignore sanitize them, the same way as for the
other binary writers and dump().
Signed-off-by: Niels Lohmann <mail@nlohmann.me>