Added method ReadConfigNoNil

This patch adds a new method called ReadConfigNoNil that will read in a
configuration file, but only set existing keys if they exist in the
provided file.

viper.go

@@ -805,12 +805,43 @@ func (v *Viper) ReadInConfig() error {
 	return v.unmarshalReader(bytes.NewReader(file), v.config)
 }
 
+// Viper will discover and load the configuration file from disk
+// and key/value stores, searching in one of the defined paths.
+// This method will only set existing keys if they exist in the
+// provided file.
+func ReadInConfigNoNil() error { return v.ReadInConfigNoNil() }
+func (v *Viper) ReadInConfigNoNil() error {
+	jww.INFO.Println("Attempting to read in config file")
+	if !stringInSlice(v.getConfigType(), SupportedExts) {
+		return UnsupportedConfigError(v.getConfigType())
+	}
+
+	file, err := ioutil.ReadFile(v.getConfigFile())
+	if err != nil {
+		return err
+	}
+
+	return v.ReadConfigNoNil(bytes.NewReader(file))
+}
+
+// Viper will read a configuration file, setting existing keys to nil if the
+// key does not exist in the file.
 func ReadConfig(in io.Reader) error { return v.ReadConfig(in) }
 func (v *Viper) ReadConfig(in io.Reader) error {
 	v.config = make(map[string]interface{})
 	return v.unmarshalReader(in, v.config)
 }
 
+// Viper will read a configuration file, but only set existing keys if they
+// exist in the provided file.
+func ReadConfigNoNil(in io.Reader) error { return v.ReadConfigNoNil(in) }
+func (v *Viper) ReadConfigNoNil(in io.Reader) error {
+	if v.config == nil {
+		v.config = make(map[string]interface{})
+	}
+	return v.unmarshalReader(in, v.config)
+}
+
 // func ReadBufConfig(buf *bytes.Buffer) error { return v.ReadBufConfig(buf) }
 // func (v *Viper) ReadBufConfig(buf *bytes.Buffer) error {
 // 	v.config = make(map[string]interface{})

viper_test.go

@@ -37,6 +37,13 @@ eyes : brown
 beard: true
 `)
 
+var yamlExample2 = []byte(`Hacker: false
+name: robert
+age: 40
+eyes : blue
+height: 5' 8"
+`)
+
 var tomlExample = []byte(`
 title = "TOML Example"
 
@@ -255,6 +262,19 @@ func TestYML(t *testing.T) {
 	assert.Equal(t, "steve", Get("name"))
 }
 
+func TestReadConfigNoNil(t *testing.T) {
+	initYAML()
+	assert.Equal(t, "steve", Get("name"))
+
+	r := bytes.NewReader(yamlExample2)
+
+	v.ReadConfigNoNil(r)
+	assert.Equal(t, "robert", Get("name"))
+	assert.Equal(t, 40, Get("age"))
+	assert.Equal(t, true, Get("beard"))
+	assert.Equal(t, "5' 8\"", Get("height"))
+}
+
 func TestJSON(t *testing.T) {
 	initJSON()
 	assert.Equal(t, "0001", Get("id"))