Function write_response 

pub fn write_response(
    status: u16,
    reason: &str,
    headers: &[(&str, &str)],
    dst: &mut [u8],
) -> Result<usize, HttpError>

Write an HTTP/1.1 response into a byte buffer. Returns bytes written.

Returns HttpError::BufferTooSmall if dst is undersized. Use response_size to compute the exact size needed.