Fix Excel export of date columns

Summary:
In D4567, I made column formatting more strict, but possibly too strict. @anjali reports date columns showing internal Excel date formats ("42391.2292", etc).

@jack, if you have a chance, can you apply this and verify the behavior with @anjali? Repro steps should be:

  - View any tasks in Maniphest.
  - Click "Export to Excel".
  - Open document in Excel.
  - Date column should show dates, not integers around 42,000.

Otherwise I'll test this locally, I just need to rebuild some dependencies first which is a bit involved.

Test Plan: None yet.

Reviewers: jack, btrahan

Reviewed By: btrahan

CC: anjali, aran

Differential Revision: https://secure.phabricator.com/D5467

src/applications/maniphest/controller/ManiphestExportController.php

@@ -181,9 +181,8 @@ final class ManiphestExportController extends ManiphestController {
     foreach ($rows as $row => $cols) {
       foreach ($cols as $col => $spec) {
         $cell_name = $this->col($col).($row + 1);
-        $sheet
-          ->setCellValue($cell_name, $spec, $return_cell = true)
-          ->setDataType(PHPExcel_Cell_DataType::TYPE_STRING);
+        $cell = $sheet
+          ->setCellValue($cell_name, $spec, $return_cell = true);
 
         if ($row == 0) {
           $sheet->getStyle($cell_name)->applyFromArray($header_format);
@@ -195,6 +194,8 @@ final class ManiphestExportController extends ManiphestController {
             ->getStyle($cell_name)
             ->getNumberFormat()
             ->setFormatCode($code);
+        } else {
+          $cell->setDataType(PHPExcel_Cell_DataType::TYPE_STRING);
         }
       }
     }