fix string being called as closure

Author: sinnbeck

src/Asserts/AssertForm.php

@@ -4,6 +4,7 @@
 
 namespace Sinnbeck\DomAssertions\Asserts;
 
+use Closure;
 use Illuminate\Support\Str;
 use Illuminate\Testing\Assert as PHPUnit;
 use PHPUnit\Framework\Assert;
@@ -72,7 +73,7 @@ protected function getAttributeFromForm(string $attribute)
 
     public function findSelect($selector = 'select', $callback = null): static
     {
-        if (is_callable($selector)) {
+        if ($selector instanceof Closure) {
             $callback = $selector;
             $selector = 'select';
         }

src/TestResponseMacros.php

@@ -33,7 +33,7 @@ public function assertElementExists(): Closure
                 Assert::fail($exception->getMessage());
             }
 
-            if (is_callable($selector)) {
+            if ($selector instanceof Closure) {
                 $callback = $selector;
                 $selector = 'body';
             }
@@ -69,7 +69,7 @@ public function assertFormExists(): Closure
                 Assert::fail($exception->getMessage());
             }
 
-            if (is_callable($selector)) {
+            if ($selector instanceof Closure) {
                 $callback = $selector;
                 $selector = 'form';
             }