saveInternal method
void
saveInternal()
Serialize workbook
Implementation
void saveInternal() {
_updateGlobalStyles();
_saveWorkbook();
_saveWorksheets();
_saveSharedString();
_saveStyles();
_saveApp(_workbook.builtInProperties);
_saveCore(_workbook.builtInProperties);
_saveContentType();
_saveTopLevelRelation();
_saveWorkbookRelation();
}