Fix clippy warning (redundant closure)

Author: chifflier

tests/readcsr.rs

@@ -74,7 +74,7 @@ fn read_csr_with_challenge_password() {
 
     // 1. Check: Parse value
     let (rem, challenge_password_from_value) =
-        Set::from_der_and_then(challenge_password_attr.value, |i| String::from_der(i))
+        Set::from_der_and_then(challenge_password_attr.value, String::from_der)
             .expect("Error parsing challenge password attribute");
     assert_eq!(challenge_password_from_value, "A challenge password");
     assert!(rem.is_empty());