Add support for Content-Disposition response header field

[konstruktiv]

PR to fix #21.

[amandasaurus]

That looks good. But rather than the argument being the entire value of the Content-Disposition header, should it be the value of the filename? I know I would probably have to constantly consult the docs to see what the correct format is. 😅

i.e. would this be a more ergonomic and nicer API:

cgi::binary_response(200, "application/octet-stream", "filename.bin", vec![1, 2]);

If the argument is set, then the header value can be constructed from format!("attachment; filename={}", …).

Is this possible? Desireable?

[konstruktiv]

That is a nicer API, but it would miss the other parts of the Content-Disposition header field's scope (display inline or as attachment, multipart/forms).

Would it be an option to accept both in the same argument? I.e. a filename or the full header string starting with "content-disposition: " in case someone needs it. Using something like this:

if let Some(cd) = content_disposition {
    const HEADER_NAME: &str = "content-disposition: ";
    let arg_cleaned: String = cd.to_lowercase().trim().to_string();        
    let cd = if arg_cleaned.starts_with(HEADER_NAME) {
        arg_cleaned.split(HEADER_NAME).collect::<String>()
    } else {
       format!("attachment; filename=\"{}\"", cd)
    };
    response = response.header(http::header::CONTENT_DISPOSITION, cd);
}

The same could be done for Content-Type.