Use class name not value to support non-Hibernate JPA vendors

With this change I got a simple Eclipselink version of the data-jpa
sample working. I'll push that when I get time to research it a bit more
(I needed to set up a Java agent so either that might be a problem
for our integration tests if we can't work around it).

Fixes gh-1268. Cherry picked onto master after (apparently) a failed
merge of commit ac2ab39a.

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

@@ -115,7 +115,7 @@ public class JpaProperties {
 
 		private Class<?> namingStrategy;
 
-		private static Class<?> DEFAULT_NAMING_STRATEGY = SpringNamingStrategy.class;
+		private static String DEFAULT_NAMING_STRATEGY = "org.springframework.boot.orm.jpa.SpringNamingStrategy";
 
 		private String ddlAuto;
 
@@ -151,7 +151,7 @@ public class JpaProperties {
 			}
 			else if (this.namingStrategy == null) {
 				result.put("hibernate.ejb.naming_strategy",
-						DEFAULT_NAMING_STRATEGY.getName());
+						DEFAULT_NAMING_STRATEGY);
 			}
 			String ddlAuto = getOrDeduceDdlAuto(existing, dataSource);
 			if (StringUtils.hasText(ddlAuto) && !"none".equals(ddlAuto)) {