Previously inserted duplicates remain after switching from ordered to unordered state

[geieredgar]

While working on #1000, I realized that we are not deduplicating already inserted duplicates when entering unordered mode:

quinn/quinn-proto/src/connection/assembler.rs

Lines 32 to 38 in 2a8c93b

	// Enter unordered mode
	let mut recvd = RangeSet::new();
	recvd.insert(0..self.bytes_read);
	for chunk in &self.data {
	recvd.insert(chunk.offset..chunk.offset + chunk.bytes.len() as u64);
	}
	self.state = State::Unordered { recvd };

With that behavior, the following test will fail:

    #[test]
    fn read_unordered() {
        let mut x = Assembler::new();
        x.insert(0, Bytes::from_static(b"abc"), 3);
        x.insert(0, Bytes::from_static(b"abc"), 3);
        x.ensure_ordering(false).unwrap();
        assert_eq!(
            x.read(3, false),
            Some(Chunk::new(0, Bytes::from_static(b"abc")))
        );
        assert_eq!(x.read(3, false), None);
    }

Is this intentional?