Update unit test

Author: richardwilly98

Diff for: src/test/java/test/elasticsearch/plugin/river/mongodb/simple/RiverMongoDropCollectionTest.java

@@ -103,8 +103,11 @@ public void testDropCollection() throws Throwable {
 							.prepareTypesExists(getIndex())
 							.setTypes(getDatabase()).execute().actionGet()
 							.isExists(), equalTo(true));
+			String collectionName = mongoCollection.getName();
 			mongoCollection.drop();
 			Thread.sleep(wait);
+			assertThat(mongoDB.collectionExists(collectionName), equalTo(false));
+			Thread.sleep(wait);
 			refreshIndex();
 			assertThat(
 					getNode().client().admin().indices()