Allow custom headers in request

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I need to add custom headers like "X-Include". I have extended code after this line https://github.com/eosnewmedia/JSON-API-Client/blob/5769305158d7f4d50bb40213759b914b843d095b/src/JsonApiClient.php#L139

foreach($request->headers()->all() as $header => $value) {
    $httpRequest = $httpRequest->withHeader($header, $value);
}