As stated here,

The field value does not
include any leading or trailing whitespace: OWS occurring before the
first non-whitespace octet of the field value or after the last
non-whitespace octet of the field value ought to be excluded by
parsers when extracting the field value from a header field.

Here's a test case that should be passing:

#[test]
fn test_trailing_space() {
    let value = HeaderValue::from_bytes("coucou \t ".as_bytes()).unwrap();
    assert_eq!(value.to_str().unwrap(), "coucou");
}