Correct UDP checksum calculation (fixes #5)

RFC768:

> If the computed checksum is zero it is transmitted as all ones (the equivalent in one's complement arithmetic). An all zero transmitted checksum value means that the transmitter generated no checksum (for debugging or for higher level protocols that don't care).

src/udp.rs

@@ -91,7 +91,7 @@ impl<'a> UdpPdu<'a> {
     }
 
     pub fn computed_checksum(&'a self, ip: &crate::Ip) -> u16 {
-        match ip {
+        let mut csum = match ip {
             crate::Ip::Ipv4(ipv4) => util::checksum(&[
                 &ipv4.source_address().as_ref(),
                 &ipv4.destination_address().as_ref(),
@@ -108,7 +108,13 @@ impl<'a> UdpPdu<'a> {
                 &self.buffer[0..=5],
                 &self.buffer[8..],
             ]),
+        };
+
+        if csum == 0 {
+            csum = 0xFFFF;
         }
+
+        csum
     }
 
     pub fn computed_data_offset(&'a self) -> usize {