Domain validation should only consider strings

[mpesari]

I think Domain::assert() should ensure that $input is string. Consider this example:

Validator::key('name', Validator::domain())->assert(['name' => ['whoops']]);

PHP Warning:  explode() expects parameter 2 to be string, array given in .../vendor/respect/validation/library/Rules/Domain.php on line 87
PHP Warning:  count(): Parameter must be an array or an object that implements Countable in .../vendor/respect/validation/library/Rules/Domain.php on line 87
PHP Warning:  Invalid argument supplied for foreach() in .../vendor/respect/validation/library/Rules/Domain.php on line 91