proxy_get_header_map_value: early return when header not found

[eguzki]

What

When header was not found, the testing framework was panic'ing.

Returning Status::NotFound instead as expected by Proxy-wasm-sdk library.

pub fn get_map_value_bytes(map_type: MapType, key: &str) -> Result<Option<Bytes>, Status> {
    let mut return_data: *mut u8 = null_mut();
    let mut return_size: usize = 0;
    unsafe {
        match proxy_get_header_map_value(
            map_type,
            key.as_ptr(),
            key.len(),
            &mut return_data,
            &mut return_size,
        ) {
            Status::Ok => {
                if !return_data.is_null() {
                    Ok(Some(Vec::from_raw_parts(
                        return_data,
                        return_size,
                        return_size,
                    )))
                } else {
                    Ok(None)
                }
            }
            Status::NotFound => Ok(None),
            status => panic!("unexpected status: {}", status as u32),
        }
    }
}