Warning: array_unique() expects parameter 1 to be array

Problem/Motivation

The following notices are generated when editing content with an image with rotate flag enbled:

Warning: array_unique() expects parameter 1 to be array, string given in Drupal\bootstrap\Utility\Attributes->getClasses() (line 66 of themes/contrib/bootstrap/src/Utility/Attributes.php). 
Warning: array_intersect(): Expected parameter 2 to be an array, null given in Drupal\bootstrap\Utility\Attributes->hasClass() (line 100 of themes/contrib/bootstrap/src/Utility/Attributes.php). 

Steps to reproduce

1. Enable the rotate icon checkbox on any image field.
2. Edit content with that image field (make sure the user has 'rotate images' permission)
3. The mentioned PHP notices are displayed (if this enabled, ie on dev env.)

Proposed resolution

The problem is located in the function simple_image_rotate_field_widget_multivalue_form_alter(), when is defines a css class for the hidden element this module adds. The class is added as a string, but it should be an array.

-          '#attributes' => ['class' => 'rotate'],
+          '#attributes' => ['class' => ['rotate']],