fix: Value could become integer

[joshbruce]

Scenario:

A select form control where the values are numerical:

$options = [
  '0' => '0'
];

foreach ($options as $value => $content) {
  // int(0)
  var_dump($value);

  // Exception thrown because isSelected expects string
  $this->isSelected($value);
}

Modified the internal loops to pass value to strval() to convert to string type.