Allow overwriting the src file with optimized version

Fixes "Fatal error: ENOENT, no such file or directory" when src and dest are the same.

```javascript
grunt.initConfig({
	imagemin: {
		icons: {
			options: { optimizationLevel:3 },
			files: [ {
				expand:true, cwd:'public/image/',
				src:'**/*.png', dest:'public/image/'
			} ]
		}
	}
});
```