net/http: validate Host header before sending

net/http: validate Host header before sending

Verify that the Host header we send is valid.
Avoids surprising behavior such as a Host of "go.dev\r\nX-Evil:oops"
adding an X-Evil header to HTTP/1 requests.

Add a test, skip the test for HTTP/2. HTTP/2 is not vulnerable to
header injection in the way HTTP/1 is, but x/net/http2 doesn't validate
the header and will go into a retry loop when the server rejects it.
CL 506995 adds the necessary validation to x/net/http2.

For #60374

Change-Id: I05cb6866a9bead043101954dfded199258c6dd04
Reviewed-on: https://go-review.googlesource.com/c/go/+/506996
Reviewed-by: Tatiana Bradley <tatianabradley@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Damien Neil <dneil@google.com>