title: Openpyxl 2.6.0 save issue
body: <p>I have an issue while I try to save an Excel workbook with comments.
  Without any comments in the Excel file, there is no issue with my scripts. I simply use:</p>
  
  <pre><code>wb_archive = load_workbook(archive_file)
  </code></pre>
  
  <p>However, if the file I want to save has comments, it doesn't work and I have the message:  </p>
  
  <blockquote>
    <p>AttributeError: 'NoneType' object has no attribute 'read'</p>
  </blockquote>
  
  <p>So, I open the file with the method:</p>
  
  <pre><code>wb_archive = load_workbook(archive_file, keep_vba=True)
  </code></pre>
  
  <p>First run is ok, however, the second one always fails with the error:</p>
  
  <blockquote>
    <p>KeyError: "There is no item named 'xl/sharedStrings.xml' in the archive"</p>
  </blockquote>
  
  <p>Am I wrong somewhere in my code?</p>
  
  <pre><code># coding: utf8
  # !/usr/bin/env python3
  """
  Program to extract Excel data to an archive
  Program Python version 3.5
  """
  
  # Standard Library
  from pathlib import Path
  from datetime import date
  
  # External Libraries
  from openpyxl import load_workbook
  
  # Import the interface
  
  # Import the project .py files
  
  filein = "file1.xlsx"
  fileout = "file2.xlsx"
  
  def xlarchive(source_file, source_sheet, archive_file, archive_sheet, source_start_line=0, archiving_method="NEW", option_date=False):
      """
      Function to save data from an Excel Workbook (source) to another one (archive).
      Variables shall be check before calling the function.
      :param source_file: file where data are copied from (source)
      :type source_file: Path file
      :param source_sheet: name of the sheet where data are located on the source file
      :type source_sheet: string
      :param archive_file: file where data are copied to (destination)
      :type archive_file: Path file
      :param archive_sheet: name of the sheet where data have to be copied on the destination file
      :type archive_sheet: string
      :param source_start_line:
      :type source_start_line: int
      :param archiving_method: defines if the destination file has to be created
      :type archiving_method: string
      :param option_date: defines if the extraction data shall be recorded
      :type option_date: bool
      :return: None
      """
      wb_source = load_workbook(source_file)
      #keep_vba = true to avoid issue with comments
      ws_source = wb_source.get_sheet_by_name(source_sheet)
      wb_archive = load_workbook(archive_file)
      ws_archive = wb_archive.get_sheet_by_name(archive_sheet)
      if archiving_method == "NEW":
          # index of [archive_sheet] sheet
          idx = wb_archive.sheetnames.index(archive_sheet)
          # remove [ws_archive]
          wb_archive.remove(ws_archive)
          # create an empty sheet [ws_archive] using old index
          wb_archive.create_sheet(archive_sheet, idx)
          ws_archive = wb_archive.get_sheet_by_name(archive_sheet)
      # If extraction has been performed the same day, previous data will be replaced
      # Date are store in Excel under format YYYY-MM-DD HH:MM:SS.
      # extractiondate is from datetime.now().date() and its format is YYYY-MM-DD
      # Comparison thanks to string is needed
      # As Openpyxl does not enable to delete row, the below code clear data and find the first empty occurrence
      if option_date == True:
          j = 0
          for i in range(ws_archive.max_row, 1, -1):
              if str(ws_archive.cell(row=i, column=1).value)[0:10] == str(date.today()):
                  j=j+1
          ws_archive.delete_rows(ws_archive.max_row - j + 1, j)
  
      for row in ws_source.iter_rows(min_row=source_start_line):
          complete_row = []
          for item in row:
              complete_row.append(item.value)
          if option_date is True:
              complete_row.insert(0, str(date.today()))
          ws_archive.append(complete_row)
      wb_archive.save(archive_file)
  
  
  
  xlarchive(filein, "Sheet1", fileout, "Sheet1", option_date=True, archiving_method="False", source_start_line=2)
  </code></pre>