Polish: String function use should be optimized for single characters

(cherry picked from commit 49fd724)

spring-tx/src/main/java/org/springframework/transaction/interceptor/MethodMapTransactionAttributeSource.java

@@ -122,7 +122,7 @@ public class MethodMapTransactionAttributeSource
 	 */
 	public void addTransactionalMethod(String name, TransactionAttribute attr) {
 		Assert.notNull(name, "Name must not be null");
-		int lastDotIndex = name.lastIndexOf(".");
+		int lastDotIndex = name.lastIndexOf('.');
 		if (lastDotIndex == -1) {
 			throw new IllegalArgumentException("'" + name + "' is not a valid method name: format is FQN.methodName");
 		}