add default "Accept: application/json" headers in OAuth2Provider

Author: codemasher

src/Core/OAuth2Provider.php

@@ -209,6 +209,7 @@ protected function sendAccessTokenRequest(string $url, array $body):ResponseInte
 
 		$request = $this->requestFactory
 			->createRequest('POST', $url)
+			->withHeader('Accept', 'application/json')
 			->withHeader('Accept-Encoding', 'identity')
 			->withHeader('Content-Type', 'application/x-www-form-urlencoded')
 			->withBody($this->streamFactory->createStream(QueryUtil::build($body, PHP_QUERY_RFC1738)))
@@ -301,6 +302,7 @@ protected function sendClientCredentialsTokenRequest(string $url, array $body):R
 
 		$request = $this->requestFactory
 			->createRequest('POST', $url)
+			->withHeader('Accept', 'application/json')
 			->withHeader('Accept-Encoding', 'identity')
 			->withHeader('Content-Type', 'application/x-www-form-urlencoded')
 			->withBody($this->streamFactory->createStream(QueryUtil::build($body, PHP_QUERY_RFC1738)))