bug 60601: allow SXSSFITestDataProvider.writeOutAndReadBack(Workbook wb) to work on SXSSFWorkbooks and XSSFWorkbooks

git-svn-id: https://svn.apache.org/repos/asf/poi/trunk@1779433 13f79535-47bb-0310-9956-ffa450edef68

src/ooxml/testcases/org/apache/poi/xssf/SXSSFITestDataProvider.java

@@ -64,7 +64,10 @@ public final class SXSSFITestDataProvider implements ITestDataProvider {
      */
     @Override
     public XSSFWorkbook writeOutAndReadBack(Workbook wb) {
-        if(!(wb instanceof SXSSFWorkbook)) {
+        // wb is usually an SXSSFWorkbook, but must also work on an XSSFWorkbook
+        // since workbooks must be able to be written out and read back
+        // several times in succession
+        if(!(wb instanceof SXSSFWorkbook || wb instanceof XSSFWorkbook)) {
             throw new IllegalArgumentException("Expected an instance of SXSSFWorkbook");
         }