Use HeaderMap for headers in mock and response

[andrewtoth]

This uses the http types HeaderMap, HeaderName and HeaderValue where appropriate, instead of Vec<(String, String)>. This disallows invalid header names at the call site.

This is a breaking change, because the functions Mock::match_header and Mock::with_header no longer accept &str. They will accept &'static str however.

[lipanski]

Thanks @andrewtoth 🥇