WIP Remove + From Mode in BaseWriter (#2657)

BaseWriter openFileHandle opens its output file with mode `wb+`. We don't appear to attempt any reads on the file after it is opened in write mode, so `wb` should be sufficient. This may have been a factor in the need for PR #2326, and is likely to be responsible for issue #2372.

src/PhpSpreadsheet/Writer/BaseWriter.php

@@ -115,7 +115,7 @@ abstract class BaseWriter implements IWriter
             return;
         }
 
-        $mode = 'wb+';
+        $mode = 'wb';
         $scheme = parse_url($filename, PHP_URL_SCHEME);
         if ($scheme === 's3') {
             $mode = 'w';