(*Stream).Flush: reset buffer to beginning

Previously it would append to the end of the buffer instead of reusing the
now-free space.

Benchmark demonstrates the improvement, run with -benchtime=10s

    benchmark                                        old ns/op     new ns/op     delta
    Benchmark_encode_string_with_SetEscapeHTML-8     447           442           -1.12%
    Benchmark_jsoniter_large_file-8                  20998         21222         +1.07%
    Benchmark_json_large_file-8                      39593         40187         +1.50%
    Benchmark_stream_encode_big_object-8             10787         8611          -20.17%

    benchmark                                        old allocs     new allocs     delta
    Benchmark_encode_string_with_SetEscapeHTML-8     6              6              +0.00%
    Benchmark_jsoniter_large_file-8                  78             78             +0.00%
    Benchmark_json_large_file-8                      13             13             +0.00%
    Benchmark_stream_encode_big_object-8             31             0              -100.00%

    benchmark                                        old bytes     new bytes     delta
    Benchmark_encode_string_with_SetEscapeHTML-8     760           760           +0.00%
    Benchmark_jsoniter_large_file-8                  4920          4920          +0.00%
    Benchmark_json_large_file-8                      6640          6640          +0.00%
    Benchmark_stream_encode_big_object-8             10056         0             -100.00%

Fixes #438

stream.go

@@ -103,14 +103,14 @@ func (stream *Stream) Flush() error {
 	if stream.Error != nil {
 		return stream.Error
 	}
-	n, err := stream.out.Write(stream.buf)
+	_, err := stream.out.Write(stream.buf)
 	if err != nil {
 		if stream.Error == nil {
 			stream.Error = err
 		}
 		return err
 	}
-	stream.buf = stream.buf[n:]
+	stream.buf = stream.buf[:0]
 	return nil
 }
 

stream_test.go

@@ -1,8 +1,9 @@
 package jsoniter
 
 import (
-	"github.com/stretchr/testify/require"
 	"testing"
+
+	"github.com/stretchr/testify/require"
 )
 
 func Test_writeByte_should_grow_buffer(t *testing.T) {
@@ -65,5 +66,10 @@ func Test_flush_buffer_should_stop_grow_buffer(t *testing.T) {
 	writer := new(NopWriter)
 	NewEncoder(writer).Encode(make([]int, 10000000))
 	should := require.New(t)
-	should.Equal(8, writer.bufferSize)
+
+	// 512 is the internal buffer size set in NewEncoder
+	//
+	// Flush is called after each array element, so only the first 8 bytes of it
+	// is ever used, and it is never extended. Capacity remains 512.
+	should.Equal(512, writer.bufferSize)
 }