Accept strings as disabled value.

Old FormHelper was pretty permissive about disabled values. So
we can keep that going forward.

src/View/Widget/MultiCheckbox.php

@@ -197,7 +197,7 @@ class MultiCheckbox implements WidgetInterface {
 		if ($disabled === null || $disabled === false) {
 			return false;
 		}
-		if ($disabled === true) {
+		if ($disabled === true || is_string($disabled)) {
 			return true;
 		}
 		$strict = !is_numeric($key);

tests/TestCase/View/Widget/MultiCheckboxTest.php

@@ -257,6 +257,10 @@ class MultiCheckboxTest extends TestCase {
 		];
 		$this->assertTags($result, $expected);
 
+		$data['disabled'] = 'a string';
+		$result = $input->render($data);
+		$this->assertTags($result, $expected);
+
 		$data['disabled'] = ['1', '1x'];
 		$this->assertTags($result, $expected);