net/http/httptest: make easier way to create Requests for testing

[encryptio]

When http.NewRequest is called, it does not fill in (http.Request).RequestURI, but http.Server does fill in (http.Request).RequestURI, which has caused issues writing tests that use a httptest.ResponseRecorder instead of a httptest.Server.

A test case:

package issue

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

func TestNewRequestRequestURI(t *testing.T) {
    var httpServerRequestURI string
    h := http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
        httpServerRequestURI = r.RequestURI
    })

    srv := httptest.NewServer(h)
    defer srv.Close()

    req, err := http.NewRequest("GET", srv.URL+"/path", nil)
    if err != nil {
        t.Fatal(err)
    }

    newRequestRequestURI := req.RequestURI

    resp, err := http.DefaultClient.Do(req)
    if err != nil {
        t.Fatal(err)
    }
    resp.Body.Close()

    if newRequestRequestURI != httpServerRequestURI {
        t.Fatalf("NewRequest().RequestURI = %#v, but sending to server yielded %#v",
            newRequestRequestURI, httpServerRequestURI)
    }
}

Which yields:

--- FAIL: TestNewRequestRequestURI (0.00s)
    test_test.go:33: NewRequest().RequestURI = "", but sending to server yielded "/path"
FAIL
exit status 1
FAIL    issue   0.004s

On go version go1.6 linux/amd64 and go version devel +c12e1b0 Tue Mar 22 17:09:29 2016 +0000 linux/amd64.

[minux]

Working as intended (and documented).

The RequestURI field is not supposed to be set on client requests.
Quote https://golang.org/pkg/net/http/#Request:

    // RequestURI is the unmodified Request-URI of the
    // Request-Line (RFC 2616, Section 5.1) as sent by the client
    // to a server. Usually the URL field should be used instead.
    // It is an error to set this field in an HTTP client request.
    RequestURI string

And NewRequest is not designed to create server Requests:
Quote https://golang.org/pkg/net/http/#NewRequest

    NewRequest returns a Request suitable for use with Client.Do
    or Transport.RoundTrip. To create a request for use with testing
    a Server Handler use either ReadRequest or manually update the
    Request fields. See the Request type's documentation for the
    difference between inbound and outbound request fields.

[bradfitz]

For Go 1.7, I do want to make a new httptest helper to make it easier to create a Request for testing, though. I think there's already an open bug, but just in case I'll re-open this one and assign it to me.

[cespare]

@bradfitz #14199 is the existing issue. Probably okay to leave this one closed.