Guzzle always convert all my headers to lowercase - php

Question

I have a script that uses guzzle to make an API call. The api server checks for headers and it is case sensitive.
Below is an example code of mine
<?php
$headers = [
'set_headers' => [
'Connection' => 'Keep-Alive',
'Accept-Encoding' => ‘gzip’,
'Accept-Language' => ‘en_US’,
'US-Token' => '1f23a-d234s-3s45d-452g',
'AToken' => 'XXXX'
],
];
// Build HTTP request object.
$request = \GuzzleHttp\Psr7\Request( // Throws (they didn't document that properly).
'POST',
'htps://api.website.com/',
$headers,
bodystream() //StreamInterface
);
// Default request options (immutable after client creation).
$_guzzleClient = new \GuzzleHttp\Client([
'handler' => $stack, // Our middleware is now injected.
'allow_redirects' => [
'max' => 8, // Allow up to eight redirects (that's plenty).
],
'connect_timeout' => 30.0, // Give up trying to connect after 30s.
'decode_content' => true, // Decode gzip/deflate/etc HTTP responses.
'timeout' => 240.0, // Maximum per-request time (seconds).
// Tells Guzzle to stop throwing exceptions on non-"2xx" HTTP codes,
// thus ensuring that it only triggers exceptions on socket errors!
// We'll instead MANUALLY be throwing on certain other HTTP codes.
'http_errors' => false,
]);
// Add critically important options for authenticating the request.
$guzzleOptions = [
'cookies' => ($_cookieJar instanceof CookieJar ? $_cookieJar : false),
'verify' => file_exists('/etc/ssl/certs/cacert.pem') ? '/etc/ssl/certs/cacert.pem' : $_verifySSL,
'proxy' => ($_proxy !== null ? $_proxy : null),
'curl' => [
CURLOPT_HTTP_VERSION => CURL_HTTP_VERSION_2, // Make http client work with HTTP 2/0
CURLOPT_SSLVERSION => 1,
CURLOPT_SSL_VERIFYPEER => false
]
];
// Attempt the request. Will throw in case of socket errors!
$response = $_guzzleClient->send($request, $guzzleOptions); ?>
I tested the same request with cURL and it works perfectly. Is there a way I can rectify this in the guzzle php library, thanks in advance.