add HeaderValue to salvo and prelude

[prabirshrestha]

Currently need to import from hyper.

use hyper::header::HeaderValue;

res.with_header(header::CONTENT_TYPE, HeaderValue::from_static("blah")?, true)?;

[chrislearn]

Try to use &str directly:

res.with_header(header::CONTENT_TYPE, "blah", true)?;

[prabirshrestha]

That works. Not sure why I didn't try it out. Probably need to go to sleep now 😃