title: Created xlsx file delete written formula
body: <p>I create an excel file via slsxwriter and want to put there a formula. I'm doing it like this:</p>
  
  <p><code>sheet.write(2, 6, "=IF(AND(ISBLANK(J2); ISBLANK(K2); ISBLANK(L2)); \"PASSED\"; \"FAILED\")")</code></p>
  
  <p>trying also this:</p>
  
  <p><code>sheet.write_formula(2, 6, "=IF(AND(ISBLANK(J2); ISBLANK(K2); ISBLANK(L2)); \"PASSED\"; \"FAILED\")")</code></p>
  
  <p>But when I later open created xlsx file, excel want to 'repair' my file and is deleting the formula... 
  (Removed Records: Formula from /xl/worksheets/sheet1.xml part)</p>
  
  <p>When I was putting it by hand in excel it works great.</p>
  
  <p>Anyone has an idea what is going on?</p>