DATAMONGO-1578 - Add missing @Test annotation to ProjectionOperationUnitTests.

Original pull request: #398.

spring-data-mongodb/src/test/java/org/springframework/data/mongodb/core/aggregation/ProjectionOperationUnitTests.java

@@ -100,6 +100,7 @@ public class ProjectionOperationUnitTests {
 		assertThat(addClause.get(1), is((Object) 41));
 	}
 
+	@Test
 	public void arithmenticProjectionOperationWithoutAlias() {
 
 		String fieldName = "a";