Remove unnecessary json_encode when Content-Type is application/json

examples/post_json.php

@@ -8,11 +8,11 @@ use \Curl\Curl;
 //     -d "{"id":"1","content":"Hello world!","date":"2015-06-30 19:42:21"}" \
 //     "https://httpbin.org/post"
 
-$data = json_encode(array(
+$data = array(
     'id' => '1',
     'content' => 'Hello world!',
     'date' => date('Y-m-d H:i:s'),
-));
+);
 
 $curl = new Curl();
 $curl->setHeader('Content-Type', 'application/json');