length validation in KeyValuePair::encode for BytesValue

[diegocatalao]

Hello folks,

while writing unit tests for the coding module, i noticed decode validates BytesValue length against u16::MAX (Section 1.4.2), but encode doesn't. i'm not entirely sure if i'm right here (rust isnt my strongest area) but i can open a pr if you think it makes sense.

you can reproduce with this test

#[test]
fn encode_bytes_length_exceeded() {
  let mut buf = BytesMut::new();

  // you can just create a vector with 65535 + 1 bytes
  // ignoring draft-ietf-moq-transport-15 specification:
  // 
  // "Length: Only present when Type is odd.  Specifies the length of
  // the Value field in bytes.  The maximum length of a value is 2^16-1
  // bytes.  If an endpoint receives a length larger than the maximum,
  // it MUST close the session with a Protocol Violation."
  let huge_bytes = vec![0u8; u16::MAX as usize + 1];
  let kvp = KeyValuePair::new_bytes(1, huge_bytes);
  let result = kvp.encode(&mut buf);

  assert!(result.is_err(), "must rejected because bytes > 65535");
}

best

[sgodin]

@diegocatalao that observation looks correct to me. A PR sounds great, thanks. :)