net/rpc/jsonrpc: use strings.Builder

Change-Id: I251f734adbaf83aa03e3f4f37add4e116f5af093
Reviewed-on: https://go-review.googlesource.com/c/go/+/428271
Reviewed-by: Ian Lance Taylor <iant@google.com>
Reviewed-by: Michael Knyszek <mknyszek@google.com>
Run-TryBot: Ian Lance Taylor <iant@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Auto-Submit: Ian Lance Taylor <iant@google.com>

src/net/rpc/jsonrpc/all_test.go

@@ -5,7 +5,6 @@
 package jsonrpc
 
 import (
-	"bytes"
 	"encoding/json"
 	"errors"
 	"fmt"
@@ -262,7 +261,7 @@ func TestMalformedOutput(t *testing.T) {
 }
 
 func TestServerErrorHasNullResult(t *testing.T) {
-	var out bytes.Buffer
+	var out strings.Builder
 	sc := NewServerCodec(struct {
 		io.Reader
 		io.Writer