fix(net/ghttp): invalid CORS AllowOrigin parsing from referer

[yzy613]

#3986 (comment)

Example 2 has a problem.

referer := "https://example.com/path/to/page.html"
if p := gstr.PosR(referer, "/", 6); p != -1 {
	fmt.Println(referer[:p]) //https://example.com/path/to
} else {
	fmt.Println(referer)
}