Fix FixedAuthoritiesExtractor map extract bug

Fix issue where FixedAuthoritiesExtractor would fail to obtain standard
item from the map because an incorrect key was used.

Fixes gh-6396

spring-boot-autoconfigure/src/main/java/org/springframework/boot/autoconfigure/security/oauth2/resource/FixedAuthoritiesExtractor.java

@@ -79,7 +79,7 @@ public class FixedAuthoritiesExtractor implements AuthoritiesExtractor {
 		}
 		for (String key : AUTHORITY_KEYS) {
 			if (map.containsKey(key)) {
-				return map.get(map);
+				return map.get(key);
 			}
 		}
 		return map;

spring-boot-autoconfigure/src/test/java/org/springframework/boot/autoconfigure/security/oauth2/resource/FixedAuthoritiesExtractorTests.java

@@ -75,8 +75,10 @@ public class FixedAuthoritiesExtractorTests {
 
 	@Test
 	public void authoritiesAsListOfMapsWithStandardKey() {
-		this.map.put("authorities",
-				Arrays.asList(Collections.singletonMap("role", "ROLE_ADMIN")));
+		Map<String, String> map = new LinkedHashMap<String, String>();
+		map.put("role", "ROLE_ADMIN");
+		map.put("extra", "value");
+		this.map.put("authorities", Arrays.asList(map));
 		assertThat(this.extractor.extractAuthorities(this.map).toString())
 				.isEqualTo("[ROLE_ADMIN]");
 	}