net/http: document and test behavior of ServeMux with ports

net/http: document and test behavior of ServeMux with ports

Beginning on Go 1.9, ServeMux has been dropping the port number from the Host
header and in the path pattern. This commit explicitly mentions the change in
behavior and adds a simple test case to ensure consistency.

Fixes #23351.

Change-Id: I0270c8bd96cda92c13ac6437cdf66c2807b3042d
Reviewed-on: https://go-review.googlesource.com/120557
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>