Add optional native-tls support, clear up warnings for flag configurations

tests/https-agent.rs

@@ -1,7 +1,7 @@
+#[cfg(all(test, any(feature = "tls", feature = "native-tls")))]
 use std::io::Read;
 
-#[cfg(feature = "tls")]
-#[test]
+#[cfg(all(test, any(feature = "tls", feature = "native-tls")))]
 fn tls_connection_close() {
     let agent = ureq::Agent::default().build();
     let resp = agent