Allow to_u32_vec_opt to include ASNs in singleton AS_SETs

[digizeph]

Example message that fails AS paths parsing:

 A|1730905907.15|187.16.216.121|42473|52.129.48.0/24|42473 16735 53764 {395968}|INCOMPLETE|187.16.216.121||0|16735:5 16735:6102 47147:1502 47147:2002 47147:2104 47147:2300 47147:2405 47147:2604|false|53764|172.16.17.129

For the cases of AS_SET being a single ASN, we should probably handle it. See to_u32_vec_opt function here:

bgpkit-parser/src/models/bgp/attributes/aspath.rs

Lines 691 to 696 in dd12dd7

	pub fn to_u32_vec_opt(&self, dedup: bool) -> Option<Vec<u32>> {
	match self.segments.last() {
	None => None,
	Some(v) => v.to_u32_vec_opt(dedup),
	}
	}