Invalid StreamHandler Arguments

My application was incorrectly passing the StreamHandler a argument of

```
$stream = array('file://url');
$handler = new StreamHandler($stream);
``` 

And when the stream handler tried to handle a record it would cause the following error
```
Array to string conversion at /var/www/service-manager/vendor/monolog/monolog/src/Monolog/Handler/StreamHandler.php:82
```

I thought by adding an InvalidArgumentException it might show people where they are going wrong quicker
``resource`` should be lowercased.