Add FindByIndexNameSessionRepository#findByPrincipalName default method

Closes gh-1158

docs/src/test/java/docs/FindByIndexNameSessionRepositoryTests.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2014-2017 the original author or authors.
+ * Copyright 2014-2018 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.
@@ -52,9 +52,7 @@ public class FindByIndexNameSessionRepositoryTests {
 		// tag::findby-username[]
 		String username = "username";
 		Map<String, Session> sessionIdToSession = this.sessionRepository
-				.findByIndexNameAndIndexValue(
-						FindByIndexNameSessionRepository.PRINCIPAL_NAME_INDEX_NAME,
-						username);
+				.findByPrincipalName(username);
 		// end::findby-username[]
 	}
 }