DOT-4440: We added content-type: json (#9)

* DOT-4440: Fix a bug when we get data by transaction ID

* DOT-4440: code review 1

* DOT-4440: code review 2

* DOT-4440: We send data as a form of a URL encoded string

* DOT-4440: we added content-type: json

src/API/HTTPAdapter/CurlHTTPAdapter.php

@@ -35,6 +35,7 @@ protected function parseHeaders($headers)
 
     public function request(string $method, string $endpoint, array $params = [], array $headers = []): Response
     {
+        $headers["content-type"] = "application/json";
         $headers["Accept"] = "application/json";
         $method = strtolower($method);