Roles contextual filter broken with multiple arguments

In Views, enabling the multiple argument option in the Roles contextual filter breaks the query. A little digging revealed that the role IDs, which are strings, are being converted to integers, resulting in no return results.

Steps to reproduce:

1. Create a view of Users
2. Add a contextual filter for User: Roles
3. In the contextual filter configuration, enable the Allow multiple values option under More
4. Test the view by providing a role id in preview.

On displaying the query, I can see that the argument is being converted to zero, yielding no matches. In my test case, I provided an argument of 'contributor', but I can see that the query is matching against the value '0' (user__roles_value_0.roles_target_id = '0').

SELECT users_field_data.uid AS uid
FROM
{users_field_data} users_field_data
INNER JOIN {user__roles} user__roles_value_0 ON users_field_data.uid = user__roles_value_0.entity_id AND user__roles_value_0.roles_target_id = '0'
WHERE (user__roles_value_0.roles_target_id = '0') AND (users_field_data.status = '1')

I discovered that, by explicitly setting the plugin's numeric setting to FALSE in Drupal\user\Plugin\views\argument\RolesRid, the filter appears to work as intended.

$this->definition['numeric'] = FALSE;

The query now returns as intended:

SELECT users_field_data.uid AS uid
FROM
{users_field_data} users_field_data
INNER JOIN {user__roles} user__roles_value_0 ON users_field_data.uid = user__roles_value_0.entity_id AND user__roles_value_0.roles_target_id = 'contributor'
WHERE (user__roles_value_0.roles_target_id = 'contributor') AND (users_field_data.status = '1')

Steps to remedy:

1. Explicitly set the numeric definition property to FALSE in the RolesRid plugin definition.

I have attached an initial patch.