CommandArguments::options() breaks multivalue options

### Steps to reproduce

Generate an exec line via something like
```
$options = [
  'multioption' => ['foo', 'bar']
];
$this->taskExec()->options($options, '=')

```
(via https://github.com/openeuropa/task-runner)

### Expected behavior

`mycommand --multioption=foo --multioption=bar`

### Actual behavior

`mycommand --multioption=Array`
