add test file

Signed-off-by: Vasiliy Tolstov <v.tolstov@unistack.org>

codec_test.go

@@ -0,0 +1,18 @@
+package json
+
+import (
+	"bytes"
+	"testing"
+)
+
+func TestReadBody(t *testing.T) {
+	s := &struct {
+		Name string
+	}{}
+	c := NewCodec()
+	b := bytes.NewReader(nil)
+	err := c.ReadBody(b, s)
+	if err != nil {
+		t.Fatal(err)
+	}
+}