DAYS360 function needs to support dates provided as strings as well as nums

git-svn-id: https://svn.apache.org/repos/asf/poi/trunk@1901365 13f79535-47bb-0310-9956-ffa450edef68

poi/src/test/java/org/apache/poi/ss/formula/functions/TestDays360.java

@@ -21,7 +21,6 @@ import static org.apache.poi.ss.util.Utils.assertDouble;
 import static org.junit.jupiter.api.Assertions.assertEquals;
 import static org.junit.jupiter.api.Assertions.assertTrue;
 
-import java.io.IOException;
 import java.util.Calendar;
 import java.util.Date;
 import java.util.Locale;