improve Headers.iter

This code is currently used in the servo branch, forgot to make a PR to master.
Hm. So, to get the string representation, since it could be raw or typed, I'm using `to_string()`, since both representations implement Show. So, this String should be the value that would have been written out to the socket...

Should this convert the value from a String to a Vec<u8>? It seems user-code could do that if needed.