grunt.log.writeflags / grunt.verbose.writeflags says functions are 'undefined'

I find this function pretty useful when debugging, but if you have a function in the object you are logging, it will say that the function is `undefined` this did confuse me a little, and it would be nice if it logged functions as well or at least said that it was defined as a function.

An example:

```
grunt.verbose.writeflags({
someAwesomeSetting: 'GruntEqualsNice',
doSomething: function () {
// Stuff
}
}, 'Options');
```

Outputs:

`Options: someAwesomeSetting="GruntEqualsNice", doSomething=undefined`