client_credentials client should not set Authorization header when ClientAuthenticationMethod.POST

Fixes gh-6911

Author: jgrandja

oauth2/oauth2-client/src/main/java/org/springframework/security/oauth2/client/endpoint/WebClientReactiveClientCredentialsTokenResponseClient.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2018 the original author or authors.
+ * Copyright 2002-2019 the original author or authors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -91,7 +91,6 @@ public Mono<OAuth2AccessTokenResponse> getTokenResponse(OAuth2ClientCredentialsG
 	private Consumer<HttpHeaders> headers(ClientRegistration clientRegistration) {
 		return headers -> {
 			headers.setContentType(MediaType.APPLICATION_FORM_URLENCODED);
-			headers.setBasicAuth(clientRegistration.getClientId(), clientRegistration.getClientSecret());
 			if (ClientAuthenticationMethod.BASIC.equals(clientRegistration.getClientAuthenticationMethod())) {
 				headers.setBasicAuth(clientRegistration.getClientId(), clientRegistration.getClientSecret());
 			}

oauth2/oauth2-client/src/test/java/org/springframework/security/oauth2/client/endpoint/WebClientReactiveClientCredentialsTokenResponseClientTests.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2018 the original author or authors.
+ * Copyright 2002-2019 the original author or authors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -98,9 +98,11 @@ public void getTokenResponseWhenPostThenSuccess() throws Exception {
 		OAuth2ClientCredentialsGrantRequest request = new OAuth2ClientCredentialsGrantRequest(registration);
 
 		OAuth2AccessTokenResponse response = this.client.getTokenResponse(request).block();
-		String body = this.server.takeRequest().getUtf8Body();
+		RecordedRequest actualRequest = this.server.takeRequest();
+		String body = actualRequest.getUtf8Body();
 
 		assertThat(response.getAccessToken()).isNotNull();
+		assertThat(actualRequest.getHeader(HttpHeaders.AUTHORIZATION)).isNull();
 		assertThat(body).isEqualTo("grant_type=client_credentials&scope=read%3Auser&client_id=client-id&client_secret=client-secret");
 	}