Fix capitalization of imported class name

Author: jeromegamez

src/Firebase/Auth/ApiClient.php

@@ -91,7 +91,7 @@ public function setCustomUserClaims(string $uid, array $claims): ResponseInterfa
 
         return $this->requestApi($url, [
             'localId' => $uid,
-            'customAttributes' => JSON::encode((object) $claims),
+            'customAttributes' => Json::encode((object) $claims),
         ]);
     }