Re-export HeaderName

[andrewtoth]

After #179 we need to re-export HeaderName so &str can be converted to HeaderName for Mock::match_header and Mock::with_header.

For example, the following won't compile because header.as_str() doesn't live for 'static:

        let header: String = "content-type".to_string();
        let mut server = mockito::Server::new();

        server.mock("GET", "/greetings").match_header(
            header.as_str(),
            "application/json",
        );

The following won't compile because the compiler can't infer what type we're trying into:

        let header: String = "content-type".to_string();
        let mut server = mockito::Server::new();

        server.mock("GET", "/greetings").match_header(
            header.as_str().try_into().unwrap(),
            "application/json",
        );

We need to explicitly convert to a HeaderName, and we can't do that unless we have the type re-exported:

        let header: String = "content-type".to_string();
        let mut server = mockito::Server::new();

        server.mock("GET", "/greetings").match_header(
            mockito::HeaderName::try_from(header).unwrap(),
            "application/json",
        );

[lipanski]

Closing this in favour of #181

I prefer that one because I'd like to avoid exposing types from other crates if possible. It also keeps things a bit more backwards compatible.