net/http: fix doc inaccuracy and typo; tighten test

Note url.Error wrapping, and s/issue/issuing/.

Fixes #3724

R=golang-dev, dsymonds
CC=golang-dev
https://golang.org/cl/6294093

src/pkg/net/http/client.go

@@ -36,7 +36,8 @@ type Client struct {
 	// following an HTTP redirect. The arguments req and via
 	// are the upcoming request and the requests made already,
 	// oldest first. If CheckRedirect returns an error, the client
-	// returns that error instead of issue the Request req.
+	// returns that error (wrapped in a url.Error) instead of
+	// issuing the Request req.
 	//
 	// If CheckRedirect is nil, the Client uses its default policy,
 	// which is to stop after 10 consecutive requests.

src/pkg/net/http/client_test.go

@@ -231,8 +231,8 @@ func TestRedirects(t *testing.T) {
 
 	checkErr = errors.New("no redirects allowed")
 	res, err = c.Get(ts.URL)
-	if e, g := "Get /?n=1: no redirects allowed", fmt.Sprintf("%v", err); e != g {
-		t.Errorf("with redirects forbidden, expected error %q, got %q", e, g)
+	if urlError, ok := err.(*url.Error); !ok || urlError.Err != checkErr {
+		t.Errorf("with redirects forbidden, expected a *url.Error with our 'no redirects allowed' error inside; got %#v (%q)", err, err)
 	}
 }