Provide example of more in-depth TLS config

Close #480

Cargo.toml

@@ -56,3 +56,7 @@ mbedtls = { version = "0.8.1" }
 [[example]]
 name = "cureq"
 required-features = ["charset", "cookies", "socks-proxy", "native-tls"]
+
+[[example]]
+name = "tls_config"
+required-features = ["tls", "native-tls"]

examples/tls_config.rs

@@ -0,0 +1,67 @@
+//! Example of how to make more in depth TLS config, like narrowing the allowed TLS versions.
+
+use std::sync::Arc;
+
+use native_tls::Protocol;
+use rustls::version::TLS12;
+use rustls::version::TLS13;
+
+pub fn main() {
+    let mut root_store = rustls::RootCertStore::empty();
+
+    // Uncomment this to use native-certs
+
+    // let certs = rustls_native_certs::load_native_certs().expect("Could not load platform certs");
+    // for cert in certs {
+    //     // Repackage the certificate DER bytes.
+    //     let rustls_cert = rustls::Certificate(cert.0);
+    //     root_store
+    //         .add(&rustls_cert)
+    //         .expect("Failed to add native certificate too root store");
+    // }
+
+    // This adds webpki_roots certs.
+    root_store.add_server_trust_anchors(webpki_roots::TLS_SERVER_ROOTS.0.iter().map(|ta| {
+        rustls::OwnedTrustAnchor::from_subject_spki_name_constraints(
+            ta.subject,
+            ta.spki,
+            ta.name_constraints,
+        )
+    }));
+
+    // This is how we narrow down the allowed TLS versions for rustls.
+    let protocol_versions = &[&TLS12, &TLS13];
+
+    // See rustls documentation for more configuration options.
+    let tls_config = rustls::ClientConfig::builder()
+        .with_safe_default_cipher_suites()
+        .with_safe_default_kx_groups()
+        .with_protocol_versions(protocol_versions)
+        .unwrap()
+        .with_root_certificates(root_store)
+        .with_no_client_auth();
+
+    // Build a ureq agent with the rustls config.
+    let agent1 = ureq::builder().tls_config(Arc::new(tls_config)).build();
+
+    let response1 = agent1.get("https://httpbin.org/get").call().unwrap();
+    assert!(response1.status() == 200);
+
+    ////////////////////////////////////////////////////////////
+
+    // Narrow the accepted TLS versions for native-tls
+    // See native-tls documentation for more configuration options.
+    let tls_connector = native_tls::TlsConnector::builder()
+        .min_protocol_version(Some(Protocol::Tlsv12))
+        .max_protocol_version(Some(Protocol::Tlsv12))
+        .build()
+        .unwrap();
+
+    // Build a ureq agent with the native-tls config.
+    let agent2 = ureq::builder()
+        .tls_connector(Arc::new(tls_connector))
+        .build();
+
+    let response2 = agent2.get("https://httpbin.org/get").call().unwrap();
+    assert!(response2.status() == 200);
+}