net/http: use consistent IDNA processing for dial target and Host header

[kingljl]

Changes:

• internal/httpcommon/request.go: use idna.Lookup.ToASCII (UTS Replaced link in comment for gzip/deflate with valid one #46
  with mapping) when computing the Host / :authority header. Add the
  idnaASCIIHostPort helper.
• http2/transport_common.go: use idna.Lookup.ToASCII in authorityAddr
  for the HTTP/2 dial target, so the connection pool key, dialed
  address, and header agree.
• Add TestEncodeHeadersIDNAMapping and TestAuthorityAddrIDNAMapping.

Fixes golang/go#80417

[gopherbot]

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