Reverts the is_string check introduced in 2.16.0 for validation gro…

…ups as it breaks existing behaviour for form collections.

Signed-off-by: George Steel <[email protected]>

src/BaseInputFilter.php

@@ -497,7 +497,7 @@ public function getMessages()
     protected function validateValidationGroup(array $inputs)
     {
         foreach ($inputs as $name) {
-            if (! is_string($name) || ! array_key_exists($name, $this->inputs)) {
+            if (! array_key_exists($name, $this->inputs)) {
                 throw new Exception\InvalidArgumentException(sprintf(
                     'setValidationGroup() expects a list of valid input names; "%s" was not found',
                     (string) $name