Polish Authorization Server Sample

Change primary user's username to "subject" to align with
oauth2resourceserver sample.

samples/boot/oauth2authorizationserver/src/test/java/sample/OAuth2AuthorizationServerApplicationTests.java

@@ -47,7 +47,7 @@ public class OAuth2AuthorizationServerApplicationTests {
 
 		this.mvc.perform(post("/oauth/token")
 			.param("grant_type", "password")
-			.param("username", "user")
+			.param("username", "subject")
 			.param("password", "password")
 			.header("Authorization", "Basic cmVhZGVyOnNlY3JldA=="))
 				.andExpect(status().isOk());