net/http: normalize empty port in URL.Host's ":port"

- 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/22140
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/net/http/request.go

@@ -660,6 +660,8 @@ func NewRequest(method, urlStr string, body io.Reader) (*Request, error) {
 	if !ok && body != nil {
 		rc = ioutil.NopCloser(body)
 	}
+	// The host's colon:port should be normalized. See Issue 14836.
+	u.Host = removeEmptyPort(u.Host)
 	req := &Request{
 		Method:     method,
 		URL:        u,