Fixed header case normalization and content type checks

Signed-off-by: takuyanakazawa <[email protected]>

Author: tn2222

Diff for: src/OpenSearch/Serializers/SmartSerializer.php

@@ -93,8 +93,9 @@ private function isJson(array $headers): bool
         }
 
         // Check PSR-7 headers.
-        if (array_key_exists('Content-Type', $headers)) {
-            foreach ($headers['Content-Type'] as $type) {
+        $lowercaseHeaders = array_change_key_case($headers, CASE_LOWER);
+        if (array_key_exists('content-type', $lowercaseHeaders)) {
+            foreach ($lowercaseHeaders['content-type'] as $type) {
                 if (str_contains($type, 'json')) {
                     return true;
                 }

Diff for: tests/Serializers/SmartSerializerTest.php

@@ -68,6 +68,16 @@ public function testDeserialize(): void
         $this->assertEquals('bar', $result['foo']);
     }
 
+    public function testDeserializeLowercaseContentTypeHeader(): void
+    {
+        $data = '{ "foo" : "bar" }';
+        $headers = ['content-type' => ['application/json']];
+
+        $result = $this->serializer->deserialize($data, $headers);
+
+        $this->assertEquals('bar', $result['foo']);
+    }
+
     public function testDeserializeWithLegacyContentTypeHeader(): void
     {
         $data = '{ "foo" : "bar" }';