fix enum default value not work bug

[lzwjava]

fix https://github.com/nicklockwood/FXForms/issues/339

originally ,

if (!value && self.defaultValue) {
    self.value = value = self.defaultValue;
}

when I debug into it,

so !value is false ，then not go into the block .

so I think we should recognize the default enum value 。