net/http: add cookies from jar to POST request.

The main content of this CL is a test case checking the reported
issue 3511 and a tiny fix for it.  A subsequent CL will refactor
the fix as proposed issue 3511.

Fixes #3511.

R=golang-dev, steven.hartland, bradfitz
CC=golang-dev
https://golang.org/cl/6013049

src/pkg/net/http/client.go

@@ -278,6 +278,11 @@ func (c *Client) Post(url string, bodyType string, body io.Reader) (r *Response,
 		return nil, err
 	}
 	req.Header.Set("Content-Type", bodyType)
+	if c.Jar != nil {
+		for _, cookie := range c.Jar.Cookies(req.URL) {
+			req.AddCookie(cookie)
+		}
+	}
 	r, err = send(req, c.Transport)
 	if err == nil && c.Jar != nil {
 		c.Jar.SetCookies(req.URL, r.Cookies())

src/pkg/net/http/client_test.go

@@ -256,6 +256,31 @@ var echoCookiesRedirectHandler = HandlerFunc(func(w ResponseWriter, r *Request)
 	}
 })
 
+func TestClientSendsCookieFromJar(t *testing.T) {
+	tr := &recordingTransport{}
+	client := &Client{Transport: tr}
+	client.Jar = &TestJar{perURL: make(map[string][]*Cookie)}
+	us := "http://dummy.faketld/"
+	u, _ := url.Parse(us)
+	client.Jar.SetCookies(u, expectedCookies)
+
+	client.Get(us) // Note: doesn't hit network
+	matchReturnedCookies(t, expectedCookies, tr.req.Cookies())
+
+	client.Head(us) // Note: doesn't hit network
+	matchReturnedCookies(t, expectedCookies, tr.req.Cookies())
+
+	client.Post(us, "text/plain", strings.NewReader("body")) // Note: doesn't hit network
+	matchReturnedCookies(t, expectedCookies, tr.req.Cookies())
+
+	client.PostForm(us, url.Values{}) // Note: doesn't hit network
+	matchReturnedCookies(t, expectedCookies, tr.req.Cookies())
+
+	req, _ := NewRequest("GET", us, nil)
+	client.Do(req) // Note: doesn't hit network
+	matchReturnedCookies(t, expectedCookies, tr.req.Cookies())
+}
+
 // Just enough correctness for our redirect tests. Uses the URL.Host as the
 // scope of all cookies.
 type TestJar struct {