Fix rustls feature

[reneklacan]

No description provided.

[dongri]

@reneklacan What will change with this modification? In what situations were you troubled?

[reneklacan]

@dongri sorry for not adding clarification

I previously added rustls feature to support building without native-tls (it's useful for building minimal few MB docker images via https://github.com/clux/muslrust for example)

With you recently adding tokio-tungstenite with native-tls feature, rustls flag no longer worked as expected as it would include both native-tls and rustls with rustls flag.

My change is not affecting default usage in any way, it is just fixing usage with rustls feature (it's making sure tokio-tungstenite is used with rustls-tls-webpki-roots in that case instead of native-tls)

[reneklacan]

I will add that it's generally good practice for rust libraries to support nativetls and rustls. Many people prefer rustls for minimalism and performance.