[XLS, XLSX] Use another sheet name if already exists

rows will try to export sheets with name "Sheet1" but it's going to overwrite if the sheet already exists (in fact, opening the file in wb mode will overwrite it anyway).

Tasks:

• Create a test for this scenario and fix it (could be based on plugin SQLite's table name test, which already has this feature);
• Check if the problem of opening the file as wb occurs in other plugins also and create new issues if needed.