MongoSessionConverter -> AbstractMongoSessionConverter

Using Abstract prefix follows conventions for abstract classes
beginning with Abstract. This also opens up the door to introduce
a MongoSessionConverter interface in the future.

Issue gh-17

spring-session/src/test/java/org/springframework/session/data/mongo/MongoOperationsSessionRepositoryTests.java

@@ -51,7 +51,7 @@ public class MongoOperationsSessionRepositoryTests {
 	@Mock
 	MongoOperations mongoOperations;
 	@Mock
-	MongoSessionConverter converter;
+	AbstractMongoSessionConverter converter;
 
 	MongoOperationsSessionRepository sut;