Function with_header 

pub fn with_header(raw: &[u8], name: &str, value: Option<&str>) -> Vec<u8> 

The message with one header replaced: every existing line for name (folded continuations included) is dropped, and, when value is Some and non-empty, one name: value line is written at the end of the header block. Used by edit-label; the line ending of the original is kept.