Encode bson.binary.Binary.

def _encode_binary(name, value, dummy0, dummy1):
    """Encode bson.binary.Binary."""
    subtype = value.subtype
    if subtype == 2:
        value = _PACK_INT(len(value)) + value
    return b"\x05" + name + _PACK_LENGTH_SUBTYPE(len(value), subtype) + value