Display imp for ClientIpSource - #38

Adds a Display impl for ClientIpSource, and a unit test to ensure that Display == FromStr.

Currently the test has to have the variants manually listed. I appreciate this is not ideal since new variants can be added to the enum and the test would still pass. Best alternative I can think of is to create a proc-macro which generates the test, something I didn't want to do without at least talking to you first.

My usecase should you be interested is declaring a default value for an arg with clap.

#[derive(Parser, Debug)]
pub(crate) struct Foo {
    #[arg(long, default_value_t = ClientIpSource::ConnectInfo)] // <------
    pub(crate) client_ip_source: ClientIpSource,
}

For some reason they appear to be using the ToString trait and then comparing the result of that rather than using FromStr.

Appreciate your work on this crate!