The request header does not support uppercase letters

[arsiac]

• file: src/header/name.rs
• line: 1259

if name_bytes.len() == 0 || name_bytes.len() > super::MAX_HEADER_NAME_LEN || {
    let mut i = 0;
    loop {
        if i >= name_bytes.len() {
            break false;
        } else if HEADER_CHARS_H2[name_bytes[i] as usize] == 0 {
            break true;
        }
        i += 1;
    }
} { 
    ([] as [u8; 0])[0]; // Invalid header name
}

[robjtede]

Lowercase header names are used as a normalized representation, since header names are case-insensitive in HTTP/1.x and required to be lowercase in HTTP/2.

Also see https://github.com/hyperium/http/issues/228#issuecomment-404996750