windows: ensure fs_event always provides filename

`uv_fs_event` didn't provide filename argument on Windows when a file is deleted or renamed, although it does on Linux.

Fix this by ensuring that fs_event always provides filename argument on Windows.

----

Node.js API document says:

> Providing `filename` argument in the callback is not supported on every platform (currently it's only supported on Linux and Windows). Even on supported platforms `filename` is not always guaranteed to be provided.
> 
> http://nodejs.org/docs/latest/api/fs.html#fs_filename_argument

But, it's better that we provide `filename` argument if we can, isn't it?


# How to reproduce this bug

Use following script:

```javascript
var fs = require('fs');

var monitor = fs.watch('./', function(changes, fn) {
  console.log("%s :  %s", changes, fn);
});

fs.writeFileSync('foo.txt', 'new contents');
fs.renameSync('foo.txt', 'bar.txt');
fs.unlinkSync('bar.txt');

setTimeout(function() {
  monitor.close();
}, 1000);
```


## Linux

On CentOS 6.4 x64 (Node.js v0.10.29)

```console
$ node test.js
rename :  foo.txt  # writeFile
change :  foo.txt  # writeFile
rename :  foo.txt  # rename (src)
rename :  bar.txt  # rename (dst)
rename :  bar.txt  # unlink
```

On Windows 8.1 x64 (Node.j v0.10.21)

```console
> node test.js
rename :  foo.txt  # writeFile
change :  foo.txt  # writeFile
rename :  null     # rename (src) <--- FIX THIS
rename :  bar.txt  # rename (dst)
rename :  null     # unlink       <--- FIX THIS
```