added tests for missing "/" in sheets

file_test.go

@@ -364,8 +364,8 @@ func (l *FileSuite) TestMarshalFile(c *C) {
     <Override PartName="/xl/sharedStrings.xml" ContentType="application/vnd.openxmlformats-officedocument.spreadsheetml.sharedStrings+xml"></Override>
     <Override PartName="/xl/styles.xml" ContentType="application/vnd.openxmlformats-officedocument.spreadsheetml.styles+xml"></Override>
     <Override PartName="/xl/workbook.xml" ContentType="application/vnd.openxmlformats-officedocument.spreadsheetml.sheet.main+xml"></Override>
-    <Override PartName="xl/worksheets/sheet1.xml" ContentType="application/vnd.openxmlformats-officedocument.spreadsheetml.worksheet+xml"></Override>
-    <Override PartName="xl/worksheets/sheet2.xml" ContentType="application/vnd.openxmlformats-officedocument.spreadsheetml.worksheet+xml"></Override>
+    <Override PartName="/xl/worksheets/sheet1.xml" ContentType="application/vnd.openxmlformats-officedocument.spreadsheetml.worksheet+xml"></Override>
+    <Override PartName="/xl/worksheets/sheet2.xml" ContentType="application/vnd.openxmlformats-officedocument.spreadsheetml.worksheet+xml"></Override>
     <Default Extension="rels" ContentType="application/vnd.openxmlformats-package.relationships+xml"></Default>
     <Default Extension="xml" ContentType="application/xml"></Default>
   </Types>`