title: openpyxl: merged_cell_ranges and merged_cells are both empty
body: <p>I have a newly created xlsx workbook with one sheet. A1:A2 is merged and contains the string 'test'.</p>
  
  <p>Using openpyxl (2.3.0) I can access the sheet and the cell value but <code>worksheet.merged_cell_ranges</code> and <code>worksheet.merged_cells</code> are both empty:</p>
  
  <pre class="lang-py prettyprint-override"><code>wb = workbook.worksheets[0] # ReadOnlyWorksheet: &lt;ReadOnlyWorksheet "Sheet1"&gt;
  wb.cell(row=1, column=1) # 'test'
  wb.merged_cell_ranges # list: []
  wb.merged_cells # set: set([])
  </code></pre>
  
  <p>The documentation (<a href="http://openpyxl.readthedocs.org/en/latest/api/openpyxl.worksheet.html" rel="nofollow">http://openpyxl.readthedocs.org/en/latest/api/openpyxl.worksheet.html</a>) and answers to other SO questions indicate that this is not the expected behaviour.</p>
  
  <p>Yes, the cells are definitely merged. Am I doing something wrong?</p>