Response reset size

Set size to 0 when response reset

Prevents logged size accumulating with each request

response_test.go

@@ -37,4 +37,10 @@ func TestResponse(t *testing.T) {
 	if r.Size() != int64(len(s)) {
 		t.Errorf("size should be %d", len(s))
 	}
+	
+	// reser
+	r.reset(httptest.NewRecorder())
+	if r.Size() != int64(0) {
+		t.Error("size should be 0")
+	}
 }