Fix case-insensitive boolean deserialization

The visitBoolean() method only accepts lowercase "true"/"false" but XML APIs (notably .NET-based services) commonly return "True"/"False".

Author: NikoGrano

src/XmlDeserializationVisitor.php

@@ -142,7 +142,7 @@ public function visitBoolean($data, array $type): bool
     {
         $this->assertValueCanBeCastToString($data);
 
-        $data = (string) $data;
+        $data = strtolower((string) $data);
 
         if ('true' === $data || '1' === $data) {
             return true;