$ cat slash.go
package main

import (
        "log"
        "net/http"
        "net/http/httptest"
)

func main() {
        ts := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
                log.Printf("Got request for %q", r.RequestURI)
        }))
        defer ts.Close()
        req, _ := http.NewRequest("GET", ts.URL, nil)

        // Try setting Opaque to just a path.

        req.URL.Opaque = "/%2F/%2F/%2F/"
        log.Printf("1. Request URL String = %q", req.URL)
        _, err := http.DefaultClient.Do(req)
        if err != nil {
                log.Fatalf("Do: %v", err)
        }

        println("")

        // Try setting Opaque to just //host/path, so it'll String-ify
        // to a valid full URL (as required for App Engine's urlfetch)

        req.URL.Opaque = "//why-hello-there/%2F/%2F/%2F/"
        log.Printf("2. Request URL String = %q", req.URL)
        _, err = http.DefaultClient.Do(req)
        if err != nil {
                log.Fatalf("Do: %v", err)
        }
}


$ go run slash.go
1. Request URL String = "http:/%2F/%2F/%2F/"
Got request for "/%2F/%2F/%2F/"
                                                                                                                      
2. Request URL String = "http://why-hello-there/%2F/%2F/%2F/";
Got request for "//why-hello-there/%2F/%2F/%2F/"


The 1st works on for client & server with Go's *http.Transport.  (but stringifies
poorly, which means it doesn't work on App Engine).

The 2nd stringifies correctly (so it'll work with App Engine's urlfetch), but it doesn't
send the correct HTTP request with *http.Transport (it should include the scheme, if the
Opaque starts with "//")