Changed filename to be optional when saving image

Before if you wanted to use the same image you would have to pass the file path twice.

Before:
```php
$image = Image::load('fuel.png')
$image->resize('10', '10');
$image->save('fuel.png');
```

After:
```php
$image = Image::load('fuel.png')
$image->resize('10', '10');
$image->save();
```