Unit Tests for: batch changes

[enikulesku]

https://www.vinyldns.io/api/batchchange-model.html

create/get batch change has already been added (see https://github.com/vinyldns/vinyldns-java/tree/master/src/main/java/io/vinyldns/java/model/batch). We need to add some tests in https://github.com/vinyldns/vinyldns-java/blob/master/src/test/java/io/vinyldns/java/VinylDNSClientTest.java to create a basic batch change (make a batchChange with just one singleChange within it), get the batch change back and validate the fields.