net/http/httputil: use strings.Builder

Change-Id: Ifb51413894791154489bd1d4f529088958212c62
Reviewed-on: https://go-review.googlesource.com/c/go/+/428270
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Ian Lance Taylor <iant@google.com>
Auto-Submit: Ian Lance Taylor <iant@google.com>
Reviewed-by: Michael Knyszek <mknyszek@google.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>

src/net/http/httputil/dump_test.go

@@ -523,7 +523,7 @@ func TestDumpRequestOutIssue38352(t *testing.T) {
 		select {
 		case <-out:
 		case <-time.After(timeout):
-			b := &bytes.Buffer{}
+			b := &strings.Builder{}
 			fmt.Fprintf(b, "deadlock detected on iteration %d after %s with delay: %v\n", i, timeout, delay)
 			pprof.Lookup("goroutine").WriteTo(b, 1)
 			t.Fatal(b.String())

src/net/http/httputil/reverseproxy_test.go

@@ -1123,7 +1123,7 @@ func TestModifyResponseClosesBody(t *testing.T) {
 	req, _ := http.NewRequest("GET", "http://foo.tld/", nil)
 	req.RemoteAddr = "1.2.3.4:56789"
 	closeCheck := new(checkCloser)
-	logBuf := new(bytes.Buffer)
+	logBuf := new(strings.Builder)
 	outErr := errors.New("ModifyResponse error")
 	rp := &ReverseProxy{
 		Director: func(req *http.Request) {},