404 not found performance improvements

benchmark            old ns/op     new ns/op     delta
Benchmark404         737           249           -66.21%
Benchmark404Many     2330          454           -80.52%

benchmark            old allocs     new allocs     delta
Benchmark404         3              0              -100.00%
Benchmark404Many     10             0              -100.00%

benchmark            old bytes     new bytes     delta
Benchmark404         115           68            -40.87%
Benchmark404Many     235           57            -75.74%

response_writer.go

@@ -6,6 +6,7 @@ package gin
 
 import (
 	"bufio"
+	"io"
 	"net"
 	"net/http"
 )
@@ -24,6 +25,7 @@ type (
 
 		Status() int
 		Size() int
+		WriteString(string) (int, error)
 		Written() bool
 		WriteHeaderNow()
 	}
@@ -35,6 +37,8 @@ type (
 	}
 )
 
+var _ ResponseWriter = &responseWriter{}
+
 func (w *responseWriter) reset(writer http.ResponseWriter) {
 	w.ResponseWriter = writer
 	w.size = noWritten
@@ -64,6 +68,13 @@ func (w *responseWriter) Write(data []byte) (n int, err error) {
 	return
 }
 
+func (w *responseWriter) WriteString(s string) (n int, err error) {
+	w.WriteHeaderNow()
+	n, err = io.WriteString(w.ResponseWriter, s)
+	w.size += n
+	return
+}
+
 func (w *responseWriter) Status() int {
 	return w.status
 }