Remove array dependencies from url helpers

The array dependency isn't necessary anymore since the `to()` function in `UrlGenerator` uses typecasting to convert the $parameters variable into an array. This allows a user to use a single variable instead of a an array. 

```php
// For example:
url('foo/bar', $id);
secure_url('foo/bar', $id);
// Instead of:
url('foo/bar', array($id));
secure_url('foo/bar', array($id));
```

Also updated documentation to indicate a mixed type of parameters.