- Ensures that the empty port and preceeding ":"
in a URL.Host are stripped.
Normalize the empty port in a URL.Host's ":port" as
mandated by RFC 3986 Section 6.2.3 which states that:
`Likewise an explicit ":port", for which the port is empty or
the default for the scheme, is equivalent to one where the port
and its ":" delimiter are elided and thus should be
removed by scheme-based normalization.`

- Moves function `hasPort` from client.go (where it was defined but
not used directly), to http.go the common area.

Fixes #14836

Change-Id: I2067410377be9c71106b1717abddc2f8b1da1c03
Reviewed-on: https://go-review.googlesource.com/22140Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>