message: Use split_first in set_alpn
  This makes it a little cleaner.
diff: @@ -421,13 +421,13 @@ impl SecretAgent {
  // NSS inherited an idiosyncratic API as a result of having implemented NPN
  // before ALPN. For that reason, we need to put the "best" option last.
  - for v in protocols.iter().skip(1) {
  - add(v.as_ref())
  + let (first, rest) = protocols
  + .split_first()
  + .expect("at least one ALPN value needed");
  + for v in rest {
  + add(v.as_ref());
  }
  - add(protocols
  - .first()
  - .expect("at least one ALPN value needs to be provided")
  - .as_ref());
  + add(first.as_ref());
  assert_eq!(encoded_len, encoded.len());
  // Now give the result to NSS.
  