title: openpyxl: AttributeError: 'MergedCell' object attribute 'value' is read-only
body: <p>When i'm trying to fill the cell in existing .xlsx file and then save it to a new one I got message:</p>
  
  <pre><code>import openpyxl
  
  path = "/home/karol/Dokumenty/wzor.xlsx"
  wb_obj = openpyxl.load_workbook(path)
  sheet_obj = wb_obj.active
  new_protokol = sheet_obj
  
  
  firma = input("Podaj nazwe: ")
  nazwa_pliku = "Protokol odczytu"
  filename = nazwa_pliku + firma + ".xlsx"
  
  
  
  sheet_obj["C1"] = firma
  sheet_obj["D1"] = input()
  
  new_protokol.save(filename=filename)
  
  </code></pre>
  
  <pre><code>Traceback (most recent call last):
    File "/home/karol/PycharmProjects/Protokolu/Main.py", line 16, in &lt;module&gt;
      sheet_obj["C1"] = firma
    File "/home/karol/PycharmProjects/Protokolu/venv/lib/python3.7/site-packages/openpyxl/worksheet/worksheet.py", line 309, in __setitem__
      self[key].value = value
  AttributeError: 'MergedCell' object attribute 'value' is read-only
  
  Process finished with exit code 1
  </code></pre>
  
  <p>How to fix it?</p>