net/http: replace map lookup with switch for scheme port

Improve scheme port lookup by replacing map with switch, reducing overhead and improving performance.

Change-Id: I45c790da15e237d5f32c50d342b3713b98fd2ffa
GitHub-Last-Rev: 4c02e4cabf181b365fbf2b722e3051625a289527
GitHub-Pull-Request: golang/go#73422
Reviewed-on: https://go-review.googlesource.com/c/go/+/666356


Reviewed-by: Damien Neil <dneil@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: David Chase <drchase@google.com>