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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>