fixes #259: don't unnecessarily quote object properties when --screw-ie8

fixes #259

I couldn't figure out how to test it. The `compress` tests just test for structural equivalence. Here's the closest I could come (but they obviously don't work):

```js
quote_reserved_words_by_default: {
    options = {};

    input: { ({do: 0}) }
    expect: { ({"do": 0}) }
}

screw_ie8_option_allows_for_unquoted_reserved_words: {
    options = {
        screw_ie8: true,
    };

    input: { ({do: 0}) }
    expect: { ({do: 0}) }
}
```