don't throw error when config not exists

enviper.go

@@ -48,7 +48,12 @@ func New(v *viper.Viper) *Enviper {
 // The difference between enviper and viper is in automatic overriding data from file by data from env variables
 func (e *Enviper) Unmarshal(rawVal interface{}) error {
 	if err := e.Viper.ReadInConfig(); err != nil {
-		return err
+		switch err.(type) {
+		case viper.ConfigFileNotFoundError:
+			// 	do nothing
+		default:
+			return err
+		}
 	}
 	e.readEnvs(rawVal)
 	return e.Viper.Unmarshal(rawVal)

enviper_test.go

@@ -34,9 +34,25 @@ func (s *UnmarshalSuite) SetupTest() {
 func (s *UnmarshalSuite) TearDownTest()  {}
 func (s *UnmarshalSuite) TearDownSuite() {}
 
-func (s *UnmarshalSuite) TestThrowsError() {
+func (s *UnmarshalSuite) TestNotThrowsErrorWhenNoConfig() {
 	var c Config
 	e := enviper.New(viper.New())
+	s.Nil(e.Unmarshal(&c))
+	s.NotNil(c)
+}
+
+func (s *UnmarshalSuite) TestThrowsErrorWhenBrokenConfig() {
+	var c Config
+	v := viper.New()
+
+	cwd, _ := os.Getwd()
+	p := path.Join(cwd, "test_config.yaml")
+	ioutil.WriteFile(p, []byte("qwerty"), 0777)
+	defer os.Remove(p)
+
+	e := enviper.New(v)
+	e.AddConfigPath(cwd)
+	e.SetConfigName("test_config")
 	s.NotNil(e.Unmarshal(&c))
 }