msgpack5 throws when encoding NaN, contrary to spec

[ninevra]

Since #75, msgpack5 throws an error when attempting to encode NaN. The error states that NaN is not encodable in msgpack!. This appears to be contrary to the spec, which states that "Float represents a IEEE 754 double precision floating point number including NaN and Infinity".

Of note: msgpack5 already correctly deserializes NaN.