Fix WriteConfig (spf13#430, spf13#433)

viper.go

@@ -1407,11 +1407,10 @@ func (v *Viper) MergeConfigMap(cfg map[string]interface{}) error {
 // WriteConfig writes the current configuration to a file.
 func WriteConfig() error { return v.WriteConfig() }
 func (v *Viper) WriteConfig() error {
-	filename, err := v.getConfigFile()
-	if err != nil {
-		return err
+	if len(v.configPaths) < 1 {
+		return errors.New("missing configuration for 'configPath'")
 	}
-	return v.writeConfig(filename, true)
+	return v.WriteConfigAs(filepath.Join(v.configPaths[0], v.configName+"."+v.configType))
 }
 
 // SafeWriteConfig writes current configuration to file only if the file does not exist.