Allow TLS client config to be overridden

See: https://docs.rs/rustls/latest/rustls/struct.ClientConfig.html

tests/https-agent.rs

@@ -45,3 +45,96 @@ fn agent_set_cookie() {
             .unwrap()
     );
 }
+
+#[cfg(feature = "tls")]
+const BADSSL_CLIENT_CERT_PEM: &'static str = r#"Bag Attributes
+    localKeyID: 41 C3 6C 33 C7 E3 36 DD EA 4A 1F C0 B7 23 B8 E6 9C DC D8 0F
+subject=C = US, ST = California, L = San Francisco, O = BadSSL, CN = BadSSL Client Certificate
+
+issuer=C = US, ST = California, L = San Francisco, O = BadSSL, CN = BadSSL Client Root Certificate Authority
+
+-----BEGIN CERTIFICATE-----
+MIIEqDCCApCgAwIBAgIUK5Ns4y2CzosB/ZoFlaxjZqoBTIIwDQYJKoZIhvcNAQEL
+BQAwfjELMAkGA1UEBhMCVVMxEzARBgNVBAgMCkNhbGlmb3JuaWExFjAUBgNVBAcM
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+g0Y2YBH5v0xmi8sYU7weOcwynkjZARpUltBUQ0pWCF5uJsEB8uE8PPDD3c4=
+-----END CERTIFICATE-----
+Bag Attributes
+    localKeyID: 41 C3 6C 33 C7 E3 36 DD EA 4A 1F C0 B7 23 B8 E6 9C DC D8 0F
+Key Attributes: <No Attributes>
+-----BEGIN RSA PRIVATE KEY-----
+MIIEowIBAAKCAQEAxzdfEeseTs/rukjly6MSLHM+Rh0enA3Ai4Mj2sdl31x3SbPo
+en08utVhjPmlxIUdkiMG4+ffe7N+JtDLG75CaxZp9CxytX7kywooRBJsRnQhmQPc
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+m0Wqhhi8/24Sy934t5Txgkfoltg8ahkx934WjP6WWRnSAu+cf+vW
+-----END RSA PRIVATE KEY-----
+"#;
+
+#[cfg(feature = "tls")]
+#[test]
+fn tls_client_certificate() {
+    let agent = ureq::Agent::default().build();
+
+    let mut tls_config = rustls::ClientConfig::new();
+
+    let certs =
+        rustls::internal::pemfile::certs(&mut BADSSL_CLIENT_CERT_PEM.clone().as_bytes()).unwrap();
+    let key =
+        rustls::internal::pemfile::rsa_private_keys(&mut BADSSL_CLIENT_CERT_PEM.clone().as_bytes())
+            .unwrap()[0]
+            .clone();
+
+    tls_config.set_single_client_cert(certs, key).unwrap();
+    tls_config
+        .root_store
+        .add_server_trust_anchors(&webpki_roots::TLS_SERVER_ROOTS);
+
+    let resp = agent
+        .get("https://client.badssl.com/")
+        .set_tls_config(std::sync::Arc::new(tls_config))
+        .call();
+
+    assert_eq!(resp.status(), 200);
+}