Don't use a unique name for H2 database

In spring-session.sample-boot-jdbc we set
  spring.datasource.generate-unique-name=false
so the documentation's reference to "jdbc:h2:mem:testdb" is correct
(see spring-session-docs/modules/ROOT/pages/guides/boot-jdbc.adoc).

Spring Boot sets this property to "true" by default, which results in
the embedded H2 database getting a random name (instead of "testdb").

Signed-off-by: Johan Kaving <johan@kaving.se>

Author: jkaving

spring-session-samples/spring-session-sample-boot-jdbc/src/main/resources/application.properties

@@ -1,2 +1,3 @@
 spring.security.user.password=password
 spring.h2.console.enabled=true
+spring.datasource.generate-unique-name=false