Fix Utf8 validation rule

[mzur]

There was an error Call to a member function get() on string here:

core/app/Rules/Utf8.php

Line 18 in 38fb572

return mb_detect_encoding($value->get(), 'UTF-8', true) !== false;

So apparently the $value can be a string, too. Handle the case to fix the issue. Also add a test case for this.