Polish "Configure signature algorithm when using public key jwt decoder"

See gh-20523

Author: snicoll

spring-boot-project/spring-boot-autoconfigure/src/main/java/org/springframework/boot/autoconfigure/security/oauth2/resource/servlet/OAuth2ResourceServerJwtConfiguration.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2012-2019 the original author or authors.
+ * Copyright 2012-2020 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.

spring-boot-project/spring-boot-autoconfigure/src/test/java/org/springframework/boot/autoconfigure/security/oauth2/resource/servlet/OAuth2ResourceServerAutoConfigurationTests.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2012-2019 the original author or authors.
+ * Copyright 2012-2020 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.
@@ -190,6 +190,16 @@ void autoConfigurationShouldFailIfPublicKeyLocationDoesNotExist() {
 						.hasMessageContaining("Public key location does not exist"));
 	}
 
+	@Test
+	void autoConfigurationShouldFailIfAlgorithmIsInvalid() {
+		this.contextRunner
+				.withPropertyValues(
+						"spring.security.oauth2.resourceserver.jwt.public-key-location=classpath:public-key-location",
+						"spring.security.oauth2.resourceserver.jwt.jws-algorithm=NOT_VALID")
+				.run((context) -> assertThat(context).hasFailed().getFailure()
+						.hasMessageContaining("signatureAlgorithm cannot be null"));
+	}
+
 	@Test
 	void autoConfigurationWhenSetUriKeyLocationAndIssuerUriPresentShouldUseSetUri() {
 		this.contextRunner