if config type is already set, ignore file extension and use the saved config type

e.g. if viper.SetConfigType("yaml") was previously called, yaml will be used as
config format, even if the file is called XXX.conf, instead of XXXX.yaml

viper.go

@@ -1245,7 +1245,14 @@ func (v *Viper) writeConfig(filename string, force bool) error {
 	if len(ext) <= 1 {
 		return fmt.Errorf("Filename: %s requires valid extension.", filename)
 	}
-	configType := ext[1:]
+
+	var configType string
+	if v.configType == "" {
+		configType = ext[1:]
+	} else {
+		configType = v.configType
+	}
+
 	if !stringInSlice(configType, SupportedExts) {
 		return UnsupportedConfigError(configType)
 	}