Attribute values containing multidimensional data will cause an mb_srtlen exception

[TheKeymaster]

Describe the bug

Passing a multidimensional array to the Attribute instance, will cause an mb_strlen exception.

To Reproduce

// Simply create a new Attribute instance and pass it multidimensional array data:
new \FINDOLOGIC\Export\Data\Attribute(
    'attribute_with_very_long_value',
    [
        'ayy' => [
            'interesting' => 'nice'
        ]
    ]
);

Expected behavior

A clear exception indicating I have passed wrong data there, which isn't valid.

Actual behavior

An mb_strlen exception happens, which isn't really clear about what happened:

mb_strlen() expects parameter 1 to be string, array given
 /home/dom/github/findologic/libflexport/src/FINDOLOGIC/Export/Helpers/DataHelper.php:101
 /home/dom/github/findologic/libflexport/src/FINDOLOGIC/Export/Data/Attribute.php:37
 /home/dom/github/findologic/libflexport/src/FINDOLOGIC/Export/Data/Attribute.php:46
 /home/dom/github/findologic/libflexport/src/FINDOLOGIC/Export/Data/Attribute.php:28

Screenshots