Ensure that Hibernate's JtaPlatform can be customized

Previously, when JpaProperties was applying additional properties it
discarded any existing properties. This meant that custom
configuration of Hibernate's JtaPlatform was not detected and that
Boot's SpringJtaPlatform would always be configured instead.

This commit updates JpaProperties to include all of the existing
properties when it's asked to apply any additional properties.

See gh-2348
Closes gh-2359

spring-boot-autoconfigure/src/main/java/org/springframework/boot/autoconfigure/orm/jpa/JpaProperties.java

 /*
- * Copyright 2012-2014 the original author or authors.
+ * Copyright 2012-2015 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.
@@ -168,7 +168,7 @@ public class JpaProperties {
 
 		private Map<String, String> getAdditionalProperties(Map<String, String> existing,
 				DataSource dataSource) {
-			Map<String, String> result = new HashMap<String, String>();
+			Map<String, String> result = new HashMap<String, String>(existing);
 			if (!isAlreadyProvided(existing, "ejb.naming_strategy")
 					&& this.namingStrategy != null) {
 				result.put("hibernate.ejb.naming_strategy", this.namingStrategy.getName());