HTTP Agent using Host header for SNI server_name

* **Version**: v14.15.1` * **Platform**: Darwin C02DFH5SMD6M 19.6.0 Darwin Kernel Version 19.6.0: Tue Nov 10 00:10:30 PST 2020; root:xnu-6153.141.10~1/RELEASE_X86_64 x86_64 * **Subsystem**: http ### What steps will reproduce the bug? I'm not 100% sure that this is a bug, but it was surprising behavior that is different from any other http clients I've looked at (curl, python's httplib, nginx). If you make an HTTPS request specifying a `host` header that is different from the request `host`, the `servername` used for SNI will be set to the value of the `host` header, unless the `host` header contains an IP address or you explicitly pass a `servername` in the agent options. https://github.com/nodejs/node/blob/b0c0111b04201bf99fde0fe0616b9fdf1f33655d/lib/http.js#L1086-L1092 ### What is the expected behavior? This was very surprising to me as the `host` header is part of the HTTP layer, and the `servername` belongs to the TLS layer, and I have never seen a client behave this way. I would have expected the `host` (not the `host` header) to be used as the default `servername`. For example using curl: ``` $ curl -vvI https://www.google.com -H "Host: https://not-google.com" * Trying 172.217.21.164... * TCP_NODELAY set * Connected to www.google.com (172.217.21.164) port 443 (#0) * ALPN, offering h2 * ALPN, offering http/1.1 * successfully set certificate verify locations: * CAfile: /etc/ssl/cert.pem CApath: none * TLSv1.2 (OUT), TLS handshake, Client hello (1): * TLSv1.2 (IN), TLS handshake, Server hello (2): * TLSv1.2 (IN), TLS handshake, Certificate (11): * TLSv1.2 (IN), TLS handshake, Server key exchange (12): * TLSv1.2 (IN), TLS handshake, Server finished (14): * TLSv1.2 (OUT), TLS handshake, Client key exchange (16): * TLSv1.2 (OUT), TLS change cipher, Change cipher spec (1): * TLSv1.2 (OUT), TLS handshake, Finished (20): * TLSv1.2 (IN), TLS change cipher, Change cipher spec (1): * TLSv1.2 (IN), TLS handshake, Finished (20): * SSL connection using TLSv1.2 / ECDHE-…