Quote objects with numeric keys

Beautification of code with numeric keys in objects like this
```javascript
a={1:"some value"}
```
fails if "quote_keys=true" options is passed:
```
/usr/local/share/npm/lib/node_modules/uglify-js2/lib/output.js:81
        str = str.replace(/[\\\b\f\n\r\t\x22\x27\u2028\u2029\0]/g, function(s)
                  ^
TypeError: Object 1 has no method 'replace'
```
I think numeric keys should be quoted as any other, since this does not break any code (object property is still accessible by both numeric and string key).